Guy's Behavior

Guy at work / neighbor seems social but won't even say hi to me?

Hi - So this guy in my office has been ignoring me ever since I started working there even tho he seems pretty social and I haven't thought much of it but it turns out we now live in the same building... at the same floor, and when we bump into each other he won't even say hi, when we were stuck, as we had to take the elevator down together, I tried to talk but he was weirdly drawn back and didn't keep the conversation going, I mean I have done nothing to that person as we had no interaction so idg what is happening he doesn't seem vicious either, I'd like to ask what's his pb but I don't know to make it more awkward than it is now - - at work I tried to talk a little and he almost looked distressed, answered but cut it short, I'm very confused.
Guy at work / neighbor seems social but won't even say hi to me?
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