A principle that always made sense to me is that of "paying it forward". If I do something for someone, then thank you is appreciated, but doing something back in return is not prefered to paying it forward to others. I'd rather see you help someone else out, than trying to balance the book of giving and receiving between us two.
The movie the above image is a screenshot from drove that point home perfectly. It is far more natural as well, because paying it forward is like a ripple in a pond, rather than a stagnant pool that keeps all the good done in the same place.
