Scott is right on. A definition of ''self-serving'' must be established or we could all be going in different directions.
But let's just say, for the sake of conversation, that ''our'' definition of the adjective ''self-serving'' is this one: an action with one or many consequences which can possibly affect others, but always influencing one-self in a positive manner. By that definition we can now find an answer.
Humans are nothing more than animals with a universal survival instinct which always acts in accordance with what we truly desire. You may think and ask '' well, what about suicidal people?'', to which I would answer that it is what they truly want. They want to die, even though their life will end, they believe they will get a feeling of relief by ending their ''suffering''. Now, in the case of an action of charity, if the person who you gave a dollar to spat in your face, I doubt you will be giving many dollars away for a little while at least. Why is that? Simply because even though you know that giving the dollar will help him get what he needs (or in most cases wants), his reaction left you feeling as though you didn't accomplish a good deed. Therefore the satisfaction you were looking for wasn't in the ''idea'' of giving him a dollar to help him, it was in the actual pursuit of that feeling of having done seomthing good.
It is known that many wealthy people donate large sums of money to charities mostly because they think they will feel good doing so. Like being a God, so to speak.
HexHammer It could be argued that the soldier that dives on the grenade to save a comrad is doing it for himself also. Maybe he was going to die anywas because of the proximity and so at least if he saves someone doing it, he will die knowing he was courageous and that his action will be known by others.