Why do a guy and a girl catch each other eyes very often, even from far, from the back?

Why a guy and a girl catch each other eyes very often, even from far?
There is this guy I don't even know, who takes my bus. At highschool, my eyes always find his, even when I don't know he's here.
It happens very often, even in the crowd or from far. If he's behind me, I automatically turn around and we look at each other. Everywhere.

I absolutely don't know why it happens, I don't like him or anything. He's the only person with who it happens. I find it weird so I wanted to know why :)
Why do a guy and a girl catch each other eyes very often, even from far, from the back?
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