I too believe there is some meat on the bones of psychological egoism [1], the thought that altruism in its most purest sense doesn't really exist, because there is personal benefit (feeling good) to being altruistic.
There are more defensible forms of psychological egoism, but your summary ("there is personal benefit (feeling good) to being altruistic") doesn't work.
Think about it, every time you take a bite of food, you act selfishly. I guarantee you if you look hard enough you will find someone that is more starved than you are when you take that bite.... even if you, yourself are starving. How much more selfless and altruistic can you be than to give all the food you would otherwise eat for the benefit of your fellow man that otherwise would starve? (nevermind what it says about them to accept your sacrifice). How can you not be selfish when you choose to service your own need of nourishment, no matter how little you take, over choosing to feed those others that need it more.
Egoism on the other hand doesn't mean that you don't cooperate. It doesn't mean that you can't be benevolent. True, it doesn't demand any of that from you, but in the long view and all other things being equal, it's usually in your interest to cooperate and to be benevolent.
Imagine you're the only skilled baker in town. Does your community benefit more from you when you starve yourself to death, or when you keep producing food for them?