Trite as it sounds, having kid does change you, and re-aligns your priorities - especially as they start to get older. You realize pretty quickly in the process that they grow, and change, and you can never have the old - but ironically, younger - version of the child back. And there's huge bits of joy in all of it, and you want to preserve it for later, as best as you can.

Having said that - if you don't want to kids, don't have kids. Don't let people push you into it. They're great if they want them, but it's hard.