psychological egoism
Psychological egoism is the view that humans are always motivated by self-interest, even in what seem to be acts of altruism.
So, how to argue against this? There are a few from the wiki (the easiest to see is the circular logic involved), but I like this one, quoted from another forum:
Once upon a time, young ones, a great wizard trod the earth of England, and his name was Hobbes. Hobbes was cunning and wise, a man of prodigious talents, who saw deeper into the affairs of man the social creature than any who had come before. But though Hobbes' gaze penetrated the human state as sunlight in darkness, he was blind to the affairs of the human heart. In his blindness and ignorance did he conjure a monster, Psychological Egoism, and for over a hundred years the Kingdom of England was haunted by this beast.
'Twas a good and hearty man of God, Joseph Butler, who took up Logic, the immortal weapon of true heroes, which once had been wielded by Hobbes himself, and turned it against the foul dragon. "Begone, creature of Hobbes!" he cried. "I declare you impossible! If the only desire be that for pleasure, it can never be fulfilled! For pleasure is not a thing to be acquired from the world, but the product of mind, and the mind to experience pleasure must have some predisposition to produce it! These dispositions are none other than the desires, appetites, and interests which we call motives! A man feels no pleasure in benevolence unless he is benevolent!"
When the hero Butler finished his fifteen sermons, the dragon of Psychological Egoism lay dead at his feet. And across the land of Europe there was rejoicing in his deed. But in time, the deed of Butler were fell into obscurity, and the shadow of the beast once again darkened the hearts of men. Philosophers, with their knowledge of the words that once banished the beast, have retreated to their towers of ivory; they live in safety of the dragon, but forget those who live below.
So I come to you now, young ones, to tell you of those words, to teach you that the beast you fear is but a phantom, clawless, toothless, without form or fire. And so it ever was.
Relevant part bolded and italicized.
Basically, if a person feels pleasure from altruism, its because he's altruistic, not because of some weird logic saying that he's actually just trying to pleasure himself.