Why does my ex's friend stare at me so much?

My ex has this friend who stares at me a lot. When he's hanging with my ex, he doesn't stare, doesn't speak, he looks at me then looks away.

When my ex is not with him, he stares at me for a long time. I can see him staring at my butt when I walk by a window. He never says anything when he's with my ex or alone but when he's around other friends, he says hey to me when I walk by them.
Why does my ex's friend stare at me so much?
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