why do guys never hit on me?

Does it mean i'm ugly if i've never gotten hit on or asked out by a guy. I'm almost 20 and i'm not overweight and don't have bad hygiene. I take care of myself and a lot of my friends compliment me and tell me i'm pretty. But i'm invisible to guys. I've never even been complimented by a guy. I'm also pretty shy and don't make eye contact or try talking to strangers. Does this mean i'm ugly or too shy?