I don't think it necessarily has to be a selfish thing, but it often is.
I resent my father somewhat. While he was very supportive of some of my pursuits that appealed to him, he was very insulting and belittling when it came to my qualities and interest that he didn't like. My mother, on the other hand, was always able to make me and my siblings feel loved and appreciated, even while disciplining and scolding us. Even that we're all adults now, my mom still thinks of us first and herself long afterwards.
Not that it's anything more than the remotest of possibilities now, but the thought of having children is horrifying.