Infection. Your insides are all scrapped up pretty good after you push a baby out and you need time to heal. This is the same reason you can't wear tampons. Your vagina/uterus is a delicate balance in and of itself it can keep itself clean and regulated. But if you start introducing bacteria into a warm and dark environment, you are asking for trouble.

Ask your doctor why you can't have sex for 4-6 weeks. S/he'll be happy to tell you!

The doctor told me whenever I'm comfortable with having sex again. It doesn't necessarily have to be 4-6 weeks, just whenever you are comfortable.

There needs to be time to heal because the vagina is basically traumatized during natural child birth. If you feel you can have sex painlessly after 2 weeks, go ahead (AFTER you speak to your doctor)!