It shouldn't be about what you can get from others but rather what you have to give.
You give because you think it's right and when you do something right, it makes you feel good.
I do agree with you that everything would be so much better if everyone went around giving but in this world not everyone does. Since some cheat, the fairest is to deal in transactions.
Or my favorite thought of the day.
If I have but one life to live, why not live for me?
In the long term, I made a commitment to myself by procreating, that I would take care of the resulting child. My own honor, which feels good to uphold, keeps me with the child.
As far as the long term goes, I understand where you're coming from. It's a bit of a semantic game, as it's not too hard to argue that altruism is impossible and everything is ultimately done for yourself. That said, I think we differ here. I don't find the argument that everyone is equally selfish (only with different wants) compelling.
It gets complicated from here, though. You have to ask where these wants come from. Why does one person want the warmth of the needle, versus another wanting to change her child's diaper. Perhaps the ultimate selfless act is subjugating your wants in the first place, though I imagine you could argue this is also a selfish act. It could probably go on like this forever, but at some point you realize some people's circle of concern is bigger than themselves, and there must be something to this.
For instance, I can be selfish all I want but eventually, I cannot do everything myself. I will need other people to help me out in life. It's not selfish in the sense of what you learn in pre-school, it's self preservation.
I think the better question would be, what about the person that does not want to give their life for others? How will you judge them? As far as I see it, it does no good to give my life for others if I'm not going to be around to enjoy it. There's only one life to live right?