It is. There is plaque trapped between your gums and teeth that never gets removed. The point of flossing is to remove that. If you look up tutorials you will see where the floss goes (it gets a little inside your gum and hugs your teeth).

Imagine never brushing the back of your teeth, that's gonna smell bad and get infected. If you avoid doing it because it smells bad and you bleed then that's a sign you have to start doing it. Seriously, the floss is not supposed to smell bad and shouldn't get blood on it.

All I'm saying is that interproximal cavities are fucking gay and you should avoid them while you still can.