Why does this guy care what I do? He is nothing to me.?

I dont understand why this guy keeps being this way. I am now annoyed. I like him but his actions really frustrate me because they confuse me. I hate that I even let it get to me.

He is my coworker and I recently found out he has a girlfriend. A few months ago we started getting close. We would spend a lot of time together at work. We got to know eachother pretty well. It turns out we are very similar. When i found out he was taken, I cried for days so I decided to pull back and not be so close with him. I dont want him to know im hurt but at the same time, I need to put some distance between us to help me move on.

He is always looking at me now. I thought I was imagining this so I started to look back at him and he will quickly look away. He will watch me even if im just a few feet away. I always see him check up on me. If I am in another part of the store, he will scan the whole room until he sees me. If I am talking to my female coworkers, he will slowly walk by a lot and smile like if he was listening. The other day, I was talking to a guy coworker and the guy I like came and asked what we were doing. I told him he was leaving and he said so? And he was being pretty rude to him. This coworker who left used to text me a lot but about work things and one time I told the guy I like what he texted me and he came to look at my texts. It was just work things but I don't know why he did that.

Also, when other people playfully tease me, he gives them dirty looks so they apologize to me. He also cares what I think about him and has become a showoff. He once wanted me to agree with him about him being smart. I playfully said no and he seemed bothered but I said I was kidding and he just looked down and smiled. Sometimes he lifts heavy things and he makes it seem like if he's so strong. And he sings a lot now. He has been acting like such a dork lately, I can't even smile anymore.

Why would he act like this?
Why does this guy care what I do? He is nothing to me.?
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