On a basic level, kids give off joy and kids enjoy the world in ways that are long dead to us.
It is enjoyable to watch them grow and putting energy into kids when they are young feels rewarding.
Pretty much everything a kid does is important and/or rewarding biologically to the parents.
I've noticed parents don't talk about it in public, they've forgotten what it is like to be single and wrapped up in work.
Parents often only say that 'you should have kids' and 'it's great' with no further description of what it is like.
Parents are a strange phenomena, they become tribal and grounded, once a man becomes a father.