Expanding on what I said yesterday about knowing that it's good when they don't need you anymore, another truth is that it hurts you more when they leave than it hurts them.
When your children leave you it's to go to a fun babysitter, or preschool, or summer camp, or college. Someplace fun and new with things to do and learn and new friends to make and wings to stretch out and theories to test. In the meantime, you're stuck at home alone or on a plane somewhere or at your desk at work. And it's not that thrilling and you wonder how your child is doing and you miss their sweet little head.
Yes, there are times when you're dying to get away from them for a bit, when you're jumping out of your own skin from being touched or want to rip your ears out rather than hear "Whyyyyyyy???" one more time. And there are times when your kids really, really, REALLY want to be with you. But for the most part, your kids are more excited about leaving you than you are about their leaving. And when they're gone you feel more of a hole than they do.
That's good. It means you've given them enough to have confidence in themselves and to feel secure without you. So while it can sting a little that they don't feel as homesick for you as you do for them, it's all part of creating wonderful people who can go into the world with confidence.