Introduction
From the beginning, one must develop the proper motivation to listen to the Buddha's teachings. Preferably, this is to be in tune with Bodhicitta, the aspiration to attain Buddhahood so that one could be of ultimate benefit to all sentient beings.
On the basis of that, we cultivate Universal Compassion and gradually lead up to the level of Bodhicitta.
Nagarjuna on Universal Compassion Bodhicitta and Emptiness
In this regard the great Indian philosopher Nagarjuna has said that for those who want to cultivate the highest aim for oneself and others, there is no other way but through the cultivation three things:
(1) Universal Compassion, (2) Bodhicitta and (3) the cultivation of the unmistaken view of Emptiness.
He said that these three are the indispensable causes for attaining Buddhahood for oneself and others. While describing these, Nagarjuna characterizes these three indispensable causes by saying that Universal Compassion is the cause for advancement to Bodhicitta. Universal Compassion does not know of any discrimination towards sentient beings.
On the basis of this kind of compassion, one then moves ahead to develop Bodhicitta. This Bodhicitta should be so strongly built that it cannot waver at all or be moved by any kind of wayward thought. It should be firm like Mount Meru.
Along with Universal Compassion and Bodhicitta, one should develop an unmistaken understanding of Reality, known as Emptiness. Emptiness should be understood in its proper perspective relative to the Two Extremes (of eternalism and nihilism).
Cultivation of Universal Compassion is the root cause for Buddhahood. Here, Nagarjuna very clearly specifies this: It is from the root of Universal Compassion that one can then develop Bodhicitta and the unmistaken view of Emptiness.
In this regard, Acharya Dharmakirti stated that Buddhahood is to be developed through the development of the great Universal Compassion. It is this compassion which is the root cause to advancement towards Buddhahood.
According to the scriptures, Buddhahood is neither granted by someone else, nor is it held by someone to be requested to be given to you. It is rather dependent upon one's own efforts. One's own efforts are again referred back to the need to cultivate Universal Compassion, eventually developing Bodhicitta and then along with them, joining these two with the unmistaken understanding of Emptiness. Through the combination of these three, one can advance towards Buddhahood (which is the enlightened state in which one can be of ultimate benefit to oneself and others).
Take the case of our own historical Buddha. He wasn't a buddha from beginningless time. He was, like us, confused at one time. But through his cultivation of Universal Compassion, Bodhicitta and the unmistaken view of Emptiness, he eventually excelled all of us and attain the state of Buddhahood, being of ultimate reliable benefit for all sentient beings.

How to develop Universal Compassion
Universal Compassion is a genuine, spontaneous sense of concern towards all sentient beings. We wish for all sentient beings to be free from all sufferings. But this compassion can only be generated once we have gone through suffering in our own experience. Once we have developed a strong urge to be free from suffering, one extends this initial concern (that focuses on oneself) to all sentient beings.
Genuine Universal Compassion is developed within us and then serves as the stepping stone to develop Bodhicitta.

How to develop Bodhicitta
We have to first begin by experiencing or understanding the suffering nature of samsara personally. Through such exploration, we will have gained a deep enough understanding of the suffering nature of samsara. From this, there will arise a strong urge to be free from the suffering.
Following this, we can then extend such a concern towards others since others are equally steeped in such sufferings. Besides, others are more important then oneself on many grounds even if only from the point of view of numbers, others are more important than oneself.
Also, from the point of view of how other sentient beings have been kind to us from beginningless time (they have served us as mothers, friends and teachers), we can also generate a sense of closeness towards them. Thus, through such approaches, we can develop a natural sense of concern towards others when we see that they are inflicted with suffering. We achieve a sense of rejoicing towards others when we see that they are experiencing happiness. This is the way to go about developing Bodhicitta.
First, reflect on how one suffers and then extend that to others. From seeing the suffering of others, we develop a strong sense of concern in the form of compassion.
However, a doubt might arise then. No matter how strong our wish is, no matter how deep and further we develop this wish, we are unable to actually free ourselves and others.
But, we will have to come to a definite understanding: We will be unable to free everyone from suffering unless we attain Buddhahood.
Buddhahood is when we will be completely free from the obscurations. Through the complete elimination of these defilements and through the full development of the qualities of love, compassion, skillfulness and wisdom, we can eventually benefit all sentient beings in an ultimate sense, without any shortage of ability or insight on our part.
So, in this way, we will develop a conviction that it is by the attainment of Buddhahood that we can fulfill the aspiration to free all other sentient beings from their sufferings. Thus, we will wish to attain Buddhahood for the sake of all sentient beings. This is the Bodhicitta. Bodhicitta transcends all, it is the unequaled way to accumulate positivities and to overcome negativities.

In the Bodhisattvacharya Vatara by Shantideva, it is said that once we have developed this point of developing Bodhicitta, from that point onwards, one is a completely transformed being. One has become a being worthy of respect, not just by humans but also by celestial beings. All over samsara, one has excelled over all sentient beings in terms of one's spiritual achievements, spiritual developments.
From that point onwards, all of our activities, all of the actions issuing from our body, speech and mind will be conjoined and influenced by Bodhicitta.
Thus, even in our sleep and distractions, one will also be influenced by Bodhicitta. During these occasions, we will be accumulating nothing but positivities. We will gain such a strength of positivities to the point where it will outshine all of our past actions.
The positivities are likened to a big inferno or big flame that is capable of burning or consuming all kinds of negativities that we may have accumulated in the past.
At the time of developing pure Bodhicitta, we have become a completely transformed person from hence on. We will only be accumulating positivities and purifying all our negativites that we have accumulated in the past.
Bodhicitta becomes a pure way by which we advance towards the attainment of Buddhahood. Again, Bodhicitta is rooted in Universal Compassion or great compassion. In terms of the accumulation of merit, it is said that the amount of merit accumulated from an instance of developing genuine compassion and genuine Bodhicitta is incalculable, more vast than space itself.
Asanga, Maitreya, a bitch and the maggots
In this regard, we find accounts of how Acharya Asanga spent twelve years in meditation, hoping to have a vision of Buddha Maitreya. His story is an example for us.
Asanga exerted himself so much throughout those years. But still he could not have a vision of the deity Maitreya. He lost heart and came out of his meditation cave. Asanga then was walking towards town when, all of a sudden, he came across a female dog totally infected by maggots on the lower part of her body. She was being eaten away by thousands and thousands of maggots. Asanga felt so much compassion towards the maggots and towards the female dog. He was wondering how to relieve both the maggots and the female dog. The maggots themselves are so delicate that to pick them up with the hand will kill them instantly. Asanga was so strongly motivated by compassion.
To save both the maggots and the dog, Asanga cut a piece of flesh from his own body so that the maggots can have a safe place to stay after he licked the maggots off the dog with his tongue. Under such strong compassion, he reached out his tongue with the eyes closed. No matter how much closer he went, he could not touch the dog.
Asanga opened his eyes and saw Lord Maitreya there in all his glory. In shock, Asanga said to Lord Maitreya, "You seemed to lack compassion towards meditators. I have exerted so much effort over the past twelve years but I could not get close to having a vision until now!" Lord Maitreya, however, said that he has always been close to Asanga, even on the very first day of his entering retreat. It is due to the strong and thick defilements that were obscuring Acharya Asanga that he could not see Maitreya.
Thus, Asanga came to understand that the very reason why he could not get a vision was because of the defilements weighing down on his mental continuum. It was due to the strong genuine compassion towards the female dog that Asanga could clear out the obscurations.
This story shows the great strength, great effectiveness and great power of genuine compassion directed towards all sentient beings without exception. Since it is developed towards all sentient beings without any discrimination, without any differentiation, even if it is developed only for a single moment, the merit accumulated is limitless.

Butcher soars the skies
We also find an account of a butcher who lived in Lhasa. The butcher used to kill so many cows and sheep all day in his house. He killed them one by one. Once, he took a short break and went out to refresh himself. When he returned, he saw that his cleaver, used for killing the animals, was not to be found anywhere.
When he looked around, he found out that it was hidden under the earth, dug and buried there by the animals themselves. He looked at the animals. Tears were dripping from the animals' eyes and their bodies were shivering with fear.
He realized how much suffering he had been bringing to the animals, even before they were subjected to the excruciating pain of getting hacked to death. The butcher developed a strong sense of compassion towards the animals and a strong sense of aversion towards his own deeds. He was so moved with compassion that he felt he would rather end his own life than to continue with what he was doing.
The butcher went up a hill and sought to kill himself. But since that was born out of a strong sense of compassion towards sentient beings, instead of falling down and crashing to death, he actually flew to the other side of the hill.
At that time, there happened to be a meditator in the same area. The meditator happened to know the butcher person by his profession. When the meditator saw the butcher capable of flying over from one part of the hill to the other side, he thought that if even such a sinful person can do so, then he (who has spent so many years in meditation) can definitely do likewise.
So the meditator went up the same hill and threw himself down . He fell and died instantly! This shows how powerful Universal Compassion is once it is generated purely within us in its purest aspect and how effective it is in clearing away the negativities no matter how strong the load of negativities we have in our mental continuum. Developing genuine compassion and Bodhicitta is definitely worthwhile.

translated by Geshe Damdul Namgyal of Drepung Loseling
courtesy of kagyu-medialab.org