Why is a person reincarnated into a certain body? How is this decided upon?
You decide it. While you were in spirit, you made decisions about what you would like to achieve through another reincarnation. Perhaps you needed a lesson in humility after going through a life where you looked down upon everyone as inferiors. You made these decisions with the help of more advanced spirits that would position you to learn from certain experiences. Maybe you were to assist another spirit in their efforts to be kind and loving. Its a giant puzzle for our minds to contemplate but as spirits we can see much more.