Having children tends to make someone think about the future instead of one's self. It decreases the natural tendency to be self centered, because there's a self-centered little creature that screams bloody murder when hungry, cold, wet, sick, scared, etc.
Many who have children tend to assume that those who don't have them are doing so for selfish reasons, which it may be (not wanting to spend the money, don't want to lose freedom), but don't respect that decision.
I recognize that some people just don't have the impulse to have children, and some are afraid that they'll mess up or have a screwed up family life like they may have suffered from a parents' divorce.
Those decisions are personal, and calling someone selfish is rude, especially since the decision made by motivated by "I don't have kids because I don't want to ruin their lives".
In short, too many parents assume they have become saints and those who don't have kids are sinners. But there are some childless who do so for selfless reasons and some selfish parents that having kids made things worse.