A little old now, but I just noticed this comment. My recollection of the dentists words were that it increased your life expectancy by two years. That is, if you were guessing how old someone would live, knowing that they flossed would put them at two years above an average person. In other words, there isn't necessarily the slightest bit of causality in this assessment. I still find this sufficient motivation to floss, so I don't care. It would be a good idea to not take this on faith if you were interested in the truth, rather than in just motivating yourself to floss.

Edit: the other thing that helped convince me was the dentist telling me that the bleeding you get when you start isn't that you haven't developed calluses or whatever, it's that your gums are swollen/irritated due to bacteria living off your teeth. Getting past the bleeding is a sign that your flossing is working and gums are no longer irritated.