Presumably, outside of accidental pregnancy, people who would be unhappy with children don't have them and people who would be happy with children do have them. It's not really that surprising if people who choose a particular life path tend to be happy with it.
Most of the parents I'm close to are in the "wow, this actually kind of sucks" category.
Against what I hear about parenting, I'm mostly left feeling unsatisfied and unfulfilled, but what am I going to do? Maybe it'll come later.