First, when I don't floss, my gums bleed when the hygienist starts working on my mouth. Indeed, if I don't floss for 10 days or so, my gums bleed when I start flossing again. There is absolutely no question that it's noticeable when I don't floss. Given that, I have to conclude that on some level flossing is "toughening" my gums. I view that as a good thing.
Second, I floss after I have brushed my teeth. But after I've brushed my teeth, and before I floss, I rinse. So I expect my mouth to be somewhat clear of food bits. But then, when I rinse again after I've flossed, I see more leftover food bits coming out of my mouth. So I have to conclude that on some level flossing is improving my oral hygiene.
Third, I have read several times -- most recently in the 23 December NY Times article, "Tackling Inflammation to Fight Age-related Ailments -- that periodontal disease can be a source of chronic inflammation, which in turn is thought to be bad. Routine dental cleanings are recommended, and I put flossing smack square in there (see reason #2). From this perspective flossing is a simple expected value proposition. The potential benefits of flossing far outweigh anything that can possibly be gained from not flossing.
In the same way that making my bed is a great way to start the day, for me flossing is a great way to end it. After I've flossed I feel as though I've dotted the last "i" and crossed the last "t" in a day well-lived.