Why do people think you're un-happy if you're not in a relationship?

I’v been single for a while because my previous boyfriend was awful. Lately my sister has been on me about dating, I have enough to do, like work, essays….ect. She thinks I’m miserable, but I’m not. She doesn’t understand that at all, even my mother is bugging me to date. I don’t understand why they think I need to be in a relationship in order to be content. Has this ever happened to anyone and if so how did you handle it?