Why do guys always look away when we pass each other on the street?

I've noticed that whenever I am out by myself, and I see a guy (regardless of his age) walking towards me, he would look away - whether it be subtly or abruptly - just as we get close. And I'm just wondering why this is; is it because I'm ugly and they don't want to appear interested or what?
Why do guys always look away when we pass each other on the street?
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