Why does maternity leave end when the baby gets fun?

Image Gallery: Back to Work For many new mothers, the end of maternity leave comes much too soon. You know you're ready to go back to work when ...

You've finally got the diaper bag fully stocked, the baby is beginning to sleep through the night (well, mostly) and you feel like you're really starting to have this mommy thing down. Your maternity leave must be almost over.

One of the hardest parts of motherhood is tearing yourself away from your new baby to head back to work, and it seems to happen right when you and your child have hit your stride. Why does maternity leave end just when things are getting good?