I'm the same about my roommate and I feel horrible for that.
I mean I try to support her because she wants to lose weight (she's nearly 200lbs, almost twice my weight) but whenever she binges, which happens quite often, I feel better about myself because I know I'll never allow myself to become like her. I guess it's something like a self-conscious thing where you find happiness in other people's sorrows because you find it hard to make yourself happy. That doesn't necessarily mean you're sadistic, tho. It means you're human with.. unusual source of happiness, I guess?