The dominant reason for rebirth is always found in our own mind. If we look into our mind, we can see that the different planes of existence are already contained in the mind in seed form.
The dominant forces in our minds will be human states, states tied to the human world. This is the basic "tone" of our consciousness. But at times there will arise states of intense hatred which might find expression as violence or cruelty. At such moments we are constructing for ourselves a hell world. Psychologically we might be living in hell and karmically those states are the seeds of rebirth into hell.
At other times very noble thoughts will arise in us, making us feel divine or heavenly, thoughts such as supreme generosity, great kindness and compassion. With such thoughts, our world becomes very light and pure, almost like a heavenly world. These states of mind are, in fact, the seeds for rebirth in the heavenly worlds.
At times of blind desire, of brutishness, blind lust, or dull stupidity, we can see in ourselves the mind of an animal. These states are the seeds of animal existence.
We can sometimes see selfishness, possessiveness, intense clinging. At that time the mind becomes similar to the mind of a preta, an afflicted spirit, and we are planting the seed of rebirth in the preta world.
Again, there come up states of greed for power, jealousy and envy, competitiveness, the urge for power. At that time we have the mind of an asura and we lay the foundation for rebirth into the world of asuras.
So what lies behind all these planes of rebirth is the mind. Therefore the Buddha says that mind is the architect of the whole universe. We should not think of the rebirth process in terms of a human being appearing in different realms, moving from realm to realm. But rather these planes simply provide the field for the mind to work out the accumulated tendencies. The realms are only visible manifestations, the outer projections of the forces that work in the mind.