QUESTION: WHY DOES HINDUISM BELIEVE IN THE PRINCIPLE OF RE-BIRTH AND PAST LIFE KARMA AFFECTING YOU IN THE PRESENT BIRTH?

The principle of "Rebirth and Sufferings from Past Life Karma" has been written in our ancient scriptures, holy books viz. Vedas, Puran, Bhagwat Gita etc. 

Lord Brahma, the inventor of the universe, designed / created PURAN, so that it builds the base of the society which further directs the individual to their basic means of life. Purans are the books of Hindu legends and contains religious instructions. Purans narrates the history of the universe from creation to destruction, role of kings, heroes, sages, and demigods, and descriptions of Hindu philosophy.

Lord Brahma wrote the Puran consisting of 100 Crores Verses (Shloks), which was further translated by our ancient Saint Ved Vyaas (Maha Rishi Ved Vyaas).

He briefly converted those 100 Crores Verses into 4 Lakhs Verses (Shloks) and divided the same into 18 Purans.

http://basicsofhumanlife.blogspot.in/2014/03/brief-history-of-purans.html

http://basicsofhumanlife.blogspot.in/2014/03/number-of-verses-in-18-different-purans.html

http://basicsofhumanlife.blogspot.in/2014/03/the-18-primary-puranas.html