I often see this when people find out that I don't want kids; they call me selfish.

I don't really see that as a bad thing in this case. Having a kid is a life-long investment, both financially and emotionally, and one that I'm not interested in making.

Selfishness is usually bad.

But I don't see how not wanting to have kids is selfish. Its just something you don't wanna do.

Nope

It's nobody's irresponsibilty to worry about others

Considering how much of a problem overpopulation is, it is more valid to argue that people who do have kids are selfish.

You know you could always counter argue that maybe you believe you aren't up for it? Would it be selfish to have kids without caring for them properly and raising them to be decent functioning human beings, all for the sake of sex?

Yes, yes it would. So you do what you feel you should. One day your attitude and outlook might change. But, the point is, you aren't some half brained father in their early 20's dealing with 5+ kids that you don't give two shits about. Onto the question at hand now.

I say that yes, for the most part, being selfish is a bad thing, especially in this supposed modern age. But the fact is, being selfish in the past is what drove us along as a species. One tribe had land, but wanted more. So they took it. And as a result, the fittest survived. Selfishness and greed, as bad as it is in today's world, is what allowed us to survive as a species. The need, the want for more, gave our early ancestors and edge. More power, more resources, helped them survive.

I don't agree with it, but I understand that it was necessary at the time. But now, now that we've taken a few more steps as a species, selfishness is inherently bad for our survival and our progress. Because of our interconnected state, selfishness now affects not small plots of land and tribes, but the world itself.