A good answer: "Social contract". An idea from Rousseau and Hobbes, that we're good (or morals) 'cause it's good for us and for the group we live. We live in a society... and even being essencialy egoists, because our primary instinct are for our survival, so if we prejudice other 'cause of something we want, the probability of this person work in group and try do kill us are high. So we make a kind of unconscious "non-agression contract" with others, because this can increase our life period. Simple.
We're morals because it's good for us. It makes us live longer and better. If it's more "moral" than be good just because we fear a god's punishment, I don't know. But I do believe that we're not good because it's good for others, or because of goodness itself. Think... did you ever did something without have any kind of personal interest (material, psycological, spiritual, whatever)?