In order to affirm that all actions are really selfish, (and that is what you have to say if you're a p-egoist) we have to define a set of actions that would be unselfish if we were to perform them, and then argue that no, we never are unselfish in that sense. It is not enough to say that the desire in question is a desire that we have, and that we satisfy it by performing the action. Obviously that's the case. But it's can't be enough to say that Mother Teresa wants to comfort the dying, and therefore she does, so her action is selfish. What action of ours doesn't satisfy a desire, for crying out loud? This would be to define the word selfish in such a way as to trivialize it. What action could fail to satisfy a desire. What we have to argue is that the object of our actions is always some state of self and not some state of others. And once we define selfishness in that way, it seems that the evidence against psychological egoism just becomes overwhelming.
Consider someone who falls on a grenade to save his fellow soldiers. Do we have good reason to suppose that the soldier has no real interest in the safety of his fellow soldiers, and is really aiming at some good feeling he might have for falling on the grenade (for the few more seconds he's still alive?) Give me a break.