Sometimes even good people lie to the dentist. Often, these lies are on the subject of using dental floss. And, you know, if you lie to the dentist, they can tell.
Flossing can remove much of the unhealthy plaque build-up from our teeth. Each tooth has five surfaces and only using dental floss can reach a couple of them. Your toothbrush just can't get at them all.
If you don't use dental floss, you're not going to be able to clean those surfaces. Then you will have more plaque build-up that is generating acid, which causes cavities and gum irritation. Eventually you can find yourself facing gum disease. Gum disease can erode your gums and make your teeth loosen and eventually fall out. It can even cause bone in your jaw to recede.
"Beyond helping your smile, keeping your teeth clean and freshening your breath, flossing can also help you live longer according to researchers at Harvard Medical School. Evidence also exists that links poor gum health with heart disease, diabetes, respiratory disease and low birth weight babies born of non-flossing mothers."
http://reallyworks.org/blog/2010/05/30/healthy-gums-and-teeth-with-dental-flossing