It also doesn't compare with the sheer joy of having a little one who joyfully loves the heck out of you because you've bonded with each other and they feel lov

It also doesn't compare with the sheer joy of having a little one who joyfully loves the heck out of you because you've bonded with each other and they feel loved and secure with you -- you've cared for them, loved them unconditionally, been understanding, responsive, and emotionally stable with them, and patiently/consistently enforced healthy boundaries on their behavior. In a sense, one could say there's no greater joy. But there's also perhaps no greater pain when parenting gets really complex and hard. One of my kids is autistic and raising him has been incredibly difficult and painful (he's a teen now).