I think it's the very very very rare person who would do something purely for altruistic reasons. There's nothing wrong with doing something nice because it makes you feel good. I think that's honest and the 'true' reason most people do nice things... They just probably don't Realise consciously.

I am nice because I like the feeling and I think it makes me a 'good person' - whatever that means

I probably don't do it with money much, but holding doors, saying Thankyou, helping people lift things etc etc etc.... It's just nice, and feels good. And people usually smile at you, and that feels good.