You don't have to give rep back if you don't want. Rep basically is for anything that you may agree with, find hilarious or just want to give the person a pat on the back...similar to the facebook like system the way it used to be is that you could only give so much rep to one person over a 24hr period or something like that and that you had to spread around your rep giving before you could rep them again which was always referred to as the Peanut Butter rule