Hah! I wish I had a nickel for every parent who asked me this question. You're not alone! Please know that our children save their best -- AND their worst -- for those they love the most. And that's us, their parents! Studies have shown that young children keep themselves in a sort of neutral mode when they're in others's care, but really let their true selves out when they feel safe and comfortable with their mommies and daddies.
It's also part of their reaction to being left in someone else's care -- we get punished after the reunion. That's understandable -- your children wants YOU, and they have no way to control being given over to someone else for babysitting or daycare. But don't feel guilty -- being patient after a separation, and showing your child that you understand his feelings -- will help him cope better when you can't be with him. It will also show him that you respect -- and allow -- his feelings, good or bad -- which is good for his emotional growth.