Why is my male co-worker acting nervous around me even after a year?

I've been working with this guy for a little more than a year. We work in different departments but our departments can often work closely with one another. The first few months he worked with me he was always coming off very nervous. I honestly, understood the nervous behavior in the first few months because I am a naturally shy person and so when I see someone else who is a new situation acting nervous or shy, I can relate and understand. Over time, he got comfortable with me which made sense because he was feeling more comfortable in his new job environment. We even developed a really fun/joking/innocently flirty friendship at work and if we ever were both participating in after work functions we could carry over that joking banter when face-to-face.

Recently (last month or two), I've noticed he's reverting back to the nervous behavior he exhibited toward me when he first started working at my company. One example is the other day he came to my office (that I share with another co-worker) to bring some documents to my co-worker. I said "Hi _____(his name)" and his face flushed red, he didn't respond to my greeting, immediately went over to my co-worker to hand her the documents (was completely normal with her - said hello and goodbye, etc). He went to exit our office and I said "Thanks for bringing those documents over. Have a good day" and smiled at him. He looked at me, face flushed red again, smiled, didn't say anything and walked away. It's been similar type situations in recent months. He's not being rude but he's acting nervous to the point now where I feel like it is discomfort - because unlike when he first started working at my company he's moved past that "shy/nervous new employee" phase.

So, my question is why would he act this way AGAIN toward me after more than a year? Especially considering that we had reached a really good/comfortable/friendly/joking but still professional relationship at work. I am confident there is nothing I have done to offend him that would his behavior shift. I'm frustrated because I thought we were friends - at least work friends - where we could work together and be comfortable and maybe share a much needed laugh to relieve the stress. It might sound stupid but I miss him and what we had. I've received about a billion opinions from friends but really need opinions of people who don't really know me or the person I'm speaking of but are just seeing a summary of the facts.
Why is my male co-worker acting nervous around me even after a year?
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