To even the playing field maybe? If you brag about how awesome you are, you're putting the pressure on everyone else to be as awesome. Some people don't like that ...

Good point. The less-than-outstanding will look less less-desirable mate material. But they still want the benefits of altruism. So a morality develops that says, "Do good things for others (that includes me), but keep it to yourself less the rest of us look bad in comparison and fail to find suitable mates."