Happiness is not a reward for virtue: it is virtue itself.
A very common saying which may have some biological background, the feeling of "doing good" could be explained in a various of ways, some might argue that its was an evolutionary advantage for acts of altruism or "virtue" for the preservation of an entire species, or the self serving needs of an individual as they may one day be in a similar situation where we may be in need of someone else's charity. But Damasio has shown that there are centers of the brain which activate when we commit virtuous or good deeds and those areas are similar to the ones which are active when we receive pleasure!
So not only may we be encouraged by an evolutionary instinct to do so, or influenced by culture and religion to do so, but there is a biological pleasure which appears to be induced when we commit these virtuous acts so we experience pleasure doing so.
The entire point of the idea, not only reinforces how we are social creatures but are social creatures that have a pleasurable sense when we help one another.