This reminds me of a certain "friends" episode :P
On a more serious note though,there isn't a human action that is entirely altruistic.Despite the fact that their intentions might
be noble,humans always have a hidden motive that drives them to help others,even if it's done unconsciously.
A person might not expect monetary reward after doing a good deed,but he definitely enjoys the rush and good feelings he experiences after helping others,that's
partly why he does it again.
We should keep in mind here that it's normal to feel good after helping others,and it's not a harmful type of selfishness because being selfish means caring about nothing and no one besides one's personal interest,and that's not the case here.Also there's no harm done and both sides benifit from the good deed performed
.