Why do your ears hurt when you dive to the bottom of a pool?

Assuming perfect health and no other problems, the reason the ears hurt when Scuba-diving, is because of inadequate equalization of the pressure in the middle ear during decent.

All parts of the body that "contain air" need to be pressurized while diving. But other bodyparts, like the sinuses and lungs, are automatically pressurized (lung pressurization only requires you to breathe). The ears are the only part that requires action on the divers part, and only on decent.

This is a basic part of any dive-course corriculum.