Even very young children have different temperaments Why were are interested in certain things from a very young age Childhood prodigies The above give some clues to the before life. These are only things we need in this current life. Thankfully we forget most of baggage which comes with before lives. Upanishads give a good logical thought to explain it.
Upanishads explore the "subject" in us and inform that the subject in us is not the mind, body or intellect. According to the Upanishad thought. mind, body and intellect are also objects. The ultimate subject in us is the essential factor which makes body function, make the mind feel and make the intellect think. This essential factor is also called as God. The aim of human birth is supposed to be to realise that we are that God.