My coworker is shy around me, avoids speaking to me when he can, why?

He is outgoing and not the type to be insecure at work. He has initiated a collaboration with me, but through someone who works for him (instead of talking to me directly). Anytime he needs to ask me something, he gets someone else to ask for him. He basically only says hi (in a friendly manner). I am friendly, but a bit shy. I just would like us to have a normal working relationship and would like to know why he is only this way towards me? And what can I do about it?
My coworker is shy around me, avoids speaking to me when he can, why?
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