Flossing is what keeps your gums from bleeding. The more you floss the less they will bleed. The bleeding is caused by bacteria that irritate the the gum tissues (gingivitis). Many people use this an excuse not to floss but the catch 22 is that not flossing is the very thing that causes the bleeding. Don't worry about the bleeding, use it as cause to be even more aggressive in the flossing and after a few days of this they won't bleed anymore when you floss. I promise you this works every time on every one.