Why does my gum hurt and bleed when brushing my teeth?

Because...: You should see a dentist for a cleaning, possibly a deep cleaning depending on your condition. Bacteria below the gums causes irritation, bleeding and bone loss and needs to be addressed by your dentist. This is normal, so dont get too worried. But, get it checked and fixed! the longer you wait, the worse, more expensive and uncomfortable the treatment will be.

Gum disease!: Either gingivitis or periodontitis. Both cause the same symptoms as you describe. Get your teeth cleaned professionally. Take much better of your teeth on a daily basis (do excellent brushing and flossing) your dentist needs to evaulate (exam) your oral condition for proper diagnosis.