I posted before about whether adding more kids to a family constitutes a burden. Now I want to look at the other side of the cost-benefit analysis. What do we get out of having kids?
Recently researchers have attempted to show that parents are actually less happy overall than non-parents. Indeed, the day-to-day tasks of childrearing don't seem like that much fun. Between dealing with bodily functions and teaching basic manners, the moments of parental bliss (naches) seem few and far between. Parents do less "fun stuff" than non parents -- less restaurants, less theater and concerts, less sex, less vacations. The counter argument - that parents have more "meaning" in their life to compensate for the lost "fun", is hard to quantify.
I recently came across this video, which I think answers the question, although it does not primarily concern itself with family planning.
The basic thrust is that life is about connection, and connection is achieved through vulnerability. Children put you in a place of vulnerability, even while they are themselves vulnerable. Working through this mutual vulnerability not only nurtures the parent-child connection, but teaches us how to become better at forging connections with others. Another point that Brene Brown makes is that our happiness is based on feelings of self-worth. Not "self-esteem", but:
"Our job [as parents] is to say 'You know what, you're imperfect, you're wired for struggle, but you're worthy of love and belonging.'"