Why do I bleed when I brush my teeth?

Gingivitis. Bleeding from the gums is common when brushing. This can be due to gingivitis, or from brushing too vigorously. See your dentist to see if you do have gingivitis, or if you just need to change your brushing technique.
Gum inflammation. In most cases, bleeding upon brushing is the result of inflamed and/or infected gums. This is a serious condition and should be evaluated by a dentist as soon as possible.