The most commone one is of course, selfishness. I don't think the decision to have a child is about becoming less selfish at all since I know people who are parents that are VERY self focused. When many people choose to become parents, they do for a variety of reasons- never underestimate peer and familial pressure(gotta have that grandson kiddo!) If they have them and regret it, it may be that they hold a bit of a grudge towards people have not had children. It is often easier for many people to project blame and judgement on a group of people who make a lifestyle choice they don't agree with than to open themselves to a fearless and honest self examination of their own choices.