I fully admit I miss what I call the "Hallmark moments"...snuggling with my SO and our baby (this is what gets me the most), all the firsts of childhood, the moments of parental pride.
BUT, it's the fleeting moments I miss. I don't miss the sleepless nights, and far too early mornings. I don't miss the endless noise or continually forcing myself to be patient and fighting the endless battles required to properly parent. Or the worry about sending them anywhere in today's society...or what kind of world they're inheriting. I don't regret being childless...it wasn't the path for me and I wouldn't want to force someone else to live in this world. The rest is hormones and instinct.