Why is it that when we do something "good" we "feel" good about it ?

case in point, a few weeks ago I was walking to the crossing to go to work.
It has stated to rain so I opened my golfing brolly.
I hoped there may be someone waiting at the crossing so I could share my brolly.
There was, I did. We had a great 40 second chat as crossing the road.
After that, I felt good for doing it .
Why was this ?

Personally I find giving people a laugh more fulfilling, though not as fulfilling as a dozen hamburgers.   Here's what I like to do when it's raining.    Open my umbrella and keep it low, then do a huge fart and trap it in the brolly.   Then offer to share the brolly's environment with an innocent citizen.

That's hilarious fun.   They don't like to let on that someone has let off.

I put a $50 note in a blind busker's guitar case
but took change of $30 from the case.
I felt good I gave him $20.