Could there be any other reason than liking her?
Well the only other thing I could think of is he's just being friendly- it depends on how long he looked and smiled at her for. But realistically without knowing him and the girl, I'm pretty sure that's what it is. You said they don't know each other well, so they're not friends right? If they were friends, maybe you could say he was just being friendly. If they don't know each other well and he stopped what he was doing just to smile or look in her direction? He's attracted to her.
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Why is it funny?
Because there is no such thing.