This is a very important Concept. It has certain essential benefits,
so we need to have understanding of it.

I will give some hints using few examples that favours(see why not prove)
concept of past and future lives i.e. reincarnation.

Example 1 :
If two children of same parents live in same environment and gets same
treatments right from their birth. Further they both work hard (or not)
equally. Even then there abilities/interests comes out to be different in
general. This shows that there were some pre-birth abilities in children.
Example 2 :
Suppose we are listening a new song first time.Then our feelings
(and not thinking) will decide that we would like it or dislike it.
The feelings are not only limited to like and dislike, there can be many
feelings having differences in terms of degree and type. There's an
exception with case of musician and/or singer who has understanding of
music because he can think about music.
Example 3 : variation of previous instance
A person likes a song and another hates the same song. A person hate
to listen any music while another feels lonely without it.
Example 4 :
A new born comes to know that he has to cry and drink mother's milk
either consciously or unconsciously.


Conclusion :
Children at birth are not at equal level(in terms of degree and type)
they are with different interests/abilities right from birth. This
pre-existing nature results in variation.


Read more: 
Cause of sufferings
Unfairness of World
 Limits of logics
Reincarnation concept benefits