Why does gum bleed when flossing?

Infected gum: If you leave bacteria colonies on teeth and gums for more than 48 hrs, you can have an infected, bleeding gums. The fact that bleeding occur when flossing is a sign of that. Please see a dentist for a proper diagnosis.

See a dentist: You have inflamation of your gums. It is not normal or healthy for your gums to bleed while brushing or flossing. You should see your dentist and have him/her evaluate your situation.