Don't discount how many people regret not having children later in life. I'm not saying people have to have children to feel happy, but there is a non-negligible number of men and women who get to middle age and feel regret that they were not able to start a family. But by then it is almost always too late.

Edit: It's easy to look over at parents struggling with raising toddlers and think "I'm glad I don't have to do that", but once those children grow up and have lives of their own the level of fulfillment and joy those parents feel is impossible to describe to a non-parent.