Even if we take a religious person that believes in selfless acts they will be doing it because of a)community approval (b)fear of religion/following the rules (c)feeling good about themselves.
The last one pretty much sums everything up. Humans are social animals that get pleasure from helping one another. All animals do it. In biology you can find it framed under "reciprocal altruism". Although not fixed for all species same patterns seem to emerge in almost all mammals.