☛ Overzealous toothbrushing:

Brushing too hard around the gum line, or just brushing with bristles that are too hard, can erode gums.

☛ Tooth grinding (a.k.a. bruxism):

Some people grind so hard that the pressure accelerates gum erosion. In many cases, your dentist can shave down a tooth that is causing your bite to hit against another tooth. In other cases, you may need to get a customized mouth guard to wear at night (when most grinding and clenching occurs) to prevent further damage.

☛ Gum disease:

This is an infection of the gums that occurs when bacteria become lodged between the tooth and the gum. The bacteria eventually eat away at the bone and the supporting tissues at the base of the tooth. As the bone recedes, so does the surrounding gum tissue.