If we look around us, we see not only men and women, boys and girls, but also cats, dogs, birds, bugs, plants, etc. It can be seen that there are various forms of living beings in this universe.
Nothing is permanent.
Every second someone dies and someone is born. This makes us wonder what happens to those who die, and who decides what one should be born as. Jainism explains that in a very simple and sound logical way based on the karma theory. Due to the karmas associated with their souls, living beings have been going through the cycle of births and deaths in various forms since times immemorial. Unless the soul gets rid of its karmas, it will never be free from the cycle of births and deaths in different forms.