I am no expert of psychology, however, in my own understanding, I believe the feel good factor is the most promising factor, that drives us to help others. And I think, the background of that feeling comes from how you understand life, what matters to us in different times (ages), what is that you brought here and what is that you are going to take when you finally depart.
There are many people who are uber rich , and can definitely do a lot to help others directly or indirectly. However, they are busy with their own life mostly, and realize the truth when its very late.
When we are past certain age ( say 60 or more), all we need is mental piece and strength. And no amount of wealth can buy it. So if you have done something good for others, then that goodwill is going to come back and help you live a better life at that time.
After all, we have brought nothing here, nor we will take anything when we finally depart.