I think morality developed as a social instinct precisely because it benefits the individual. For humans to band together and form a society, they had to be able to account for one another's wants.

That's why, intuitively, we can recognize the pain and suffering of other beings. It's also why an important quality in what we like in others is the extent to which they have that ability. There are no examples of ethics that do not in some way redound to the benefit of the actor themselves, so even an argument based on selfishness is sufficient.