Why is he flirting with me in an immature way?

He'll bump into me, make dumb jokes about me, he says my face makes his eyes hurt, and some other stupid stuff. Why does he have to act so immature?

But when we are by ourselves he will be nice. If I ask him to put something away for me (at work) he will do it or he will look at me and smile.
Why is he flirting with me in an immature way?
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