ME versus the others

I was just wondering how it is and where we cross the line when it comes to the others. BEcause I should learn more about that.

We definitely feel happy for ourselves when anything positive happens to us. We work for ourselves, read for ourselves, have fun for ourselves, cook for ourselves, live for ourselves. However, there comes a moment when there's someone else involved. And then, we don't live only for ourselves, we don't just cook for ourselves, we don't buy and work for ourselves. Some of the things we used to do only for ourselves are not only for ourselves anymore. Whether it's a child, a boy/girlfriend, a spouse, one of our kin... My question is: do we really think of that/those person(s)altruistically or is it just because we believe in that law of the universe that the more you give the more you get back? Is there any selfish need in our helping our fellow human beings? Any selfish need in that? Or is there the selfish bit and the altruistic bit?

I cannot seem to be able to find the answer for that. Because everytime I give something, I do something for someone else it feels different. It seems that we cannot be completely and totally altruistic. It is not in our nature. If I come to think of it, back at our beginnings as hunters, there is no altruism in those human beings, in our ancestors. So, I suppose that we "grew" that feature, let's say... But it was never pat of us. So, we keep hunting and trying to make the best of OUR lives.