Guy doesn't seem to hate me... but he doesn't seem to like me either. Why would he act so unusual?

I work next to this guy and we used to talk a lot throughout the day, just two co-workers passing the time. We discovered that we share the same interests, hobbies, tastes, sense of style, etc. We would have fun working together.

Things became awkward between us when he started acting unusual for no apparent reason. He acts like we have nothing in common. If I like something, suddenly he hates it. If I find something funny, he now thinks it's stupid. I'm not sure how to explain it, but he acts like he's against me. He won't have a conversation with me but occasionally he mentions an observation he made about me. I think he looks at/watches me. He also tries to copy me and what I do. I don't think he hates me... but I don't think he likes me either. I don't understand.
Guy doesn't seem to hate me... but he doesn't seem to like me either. Why would he act so unusual?
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