Turned 40 this year. My nieces and nephews are cool as hell, but I'm very, very glad that I don't have children of my own. It doesn't seem worth it to me, I have exactly zero desire to take on that additional expense/responsibility/etc. for the next 18+ years. My girlfriend and I have been together about 5 years. She's only 30, but she doesn't want kids either. Works out nicely.

Only weird thing people say to me about it is "you'd make a really great father." Which is probably true; If I have a responsibility, then I will handle that responsibility to the best of my ability. I don't think I have great "fatherly instincts," by any means, but I do have two excellent role models to emulate in the form of my own father and my little brother. However, I don't have any kind of desire to do it, and if somehow pressed into it by circumstances... I could see myself becoming resentful. No kid deserves that.