What looks the strongest has outlived its term
The future lies with what's affirmed from under.
The quote above I stole from a report (Jaime Kalven's Garden Conversations) because it finally caused something that I've wondered about to click in.
The reason why we give back is because fate is a wheel - it's undeniable that we will not be permanent, and that someone else will come to power. That is the way of things. So when you give back to others and guide them, you make your world better and enhance the chances that the person at the top will not only be sympathetic to your causes but will treat you the same way you treated them and build in the way that you taught them. It's almost the same aspects as religious-type tenets, but more forward thinking than I've ever heard put forth. You literally build the world you live in.
Idealistic, of course. But it makes sense!