Guys stare at me in clubs but very rarely approach

Whenever I'm out I always have guys staring at me (according to a friend who goes out with me each time) but I've only ever been approached a couple of times. I don't understand why this is, it's not like I look unhappy or uncomfortable or anything. I don't go out with guy friends very often and tend to kind of split away from the rest of the group. I'm wondering if it's the way I look- ugly or intimidating maybe?

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