Usually attributed to karma, but more likely due to social reinforcement. That is, if you're nice to someone, they're likely to be nice to you because there is a greater probability you'll be nice to them again and so on back and forth. IteratedPrisonersDilemma.
Another way to look at is as follows: if N people including you put in roughly the same amount of effort, then you will get O(N) times your effort back.
Yet another way to look at is SocialCapitalism. Say you empower the otherwise unempowered such as the poor. Then each of those persons can have opportunities to grow themselves which in turn grows the society as a whole and has a effect back on you.
Yet another (cynical) way to look at is that if you give people a little help to become more efficient, they won't have to destroy the environment as much, or draw of your resources as much, or break into your house and steal your TV. -- SunirShah
[Originally from Wiki:BoomerangEffect]