Reincarnation is coming again into the body. Our mind is energy and, by the law of thermodynamics, energy cannot be destroyed. So, what happens to this energy when someone dies?
Death is almost like sleep. If you understand sleep, you’ll understand death also. While sleeping, the whole consciousness, attention, mind shrinks and one is shut off from outward experiences and moves inward into a void. When you wake up in the morning, the same energy, the same consciousness starts expanding and you wake up. If you carefully observe the mechanism, the last thought that you get just before falling asleep becomes the first thought when you wake up. This gives you a clue to reincarnation.
The mind that is full of different impressions leaves the body. But the impressions remain with the mind and wait for a proper situation for the same mind to come down. So it is the last thought that is more important. If you are happy the last moment before you leave the body, then you get a better body the next time.
Karma is that which propels reincarnation. Stronger the impression, the nature of next life could be that, predicted by that.
You might have heard of a tradition in many countries where people would name their grandsons or great-grandsons the same name as that of the grandfather. And you will find in many cases that the grandchildren behave in the same way as the grandfather, even though they are born several decades apart.
This is because when someone dies, the strongest impression in their mind is of their children or grandchildren. This strong impression creates such a karma. So, often you become like the person you hate, and you also become like the person you love. This is because love and hate are the same, one standing upside down. Distortion of love is hate. And the way to get beyond love and hate is through knowledge, through awareness. That something, which is beyond hate, is true love.