For me it's not about lacking paternal instinct. In fact, I relate to kids, I've always been comfortable around them, and they've always loved me. The reason why I don't want kids and that's okay is simply put, I don't want them. I love my life as it is, single and childless. Sure, it's a great feeling to raise a human and have them turn out okay, but that's not all there is to life. I could invent a new thing. I could take care of animals at a shelter. I could get a book published. I could feed the homeless. All of these things can foster the same emotions as raising children. Taking care of animals or the homeless? You're taking care of a human. Because of you, they may live another day. Inventing something or writing something? You know how much blood, sweat, and tears go into that? How much time goes into that? How much stress and sleepless nights?
They are all methods that give you the same satisfaction of raising a child, with far less money involved and far less of a chance of screwing up royally. Plus I've seen what happens when a parent loses a child, and what if you have to go through that? Not everyone can handle that much emotional baggage.