Why do guys always stare at me in public places?

I go to the store or am driving or am at the mall and there are always guys staring at me and I have no idea why. They always look but they don't say anything and they just stare even from the 2nd floor at the mall. I notice and quickly look away. I mean I'm really shy and I don't think I'm that pretty. I have olive tan skin, very skinny(102 lbs.), dark hair and brown eyes. I feel weird when they just stare.