Reposting for a greater amount of clarity.

2 years ago, I was a stupid 19 year old and a cute guy approached me in one of my Physics courses. We hit it off immediately. Spent significant amounts of time together, gave each other "the look", and partook in playful flirting. (Flirting never went beyond playful banter or minor physical contact- we never cuddled and I didn't put out.) Then, I found out that he has a girlfriend. Since he was in my degree program, I tried to distance myself because I felt like a scummy person for enabling him to cheat on his girlfriend. However, I saw right away I wouldn't be successful in cutting off my emotional attachments while remaining present in his life. So I unfriended him and told him that I couldn't be friends with a person who treated their SO in such a way. As far as I could tell at the time, he seemed genuinely distraught by my decision and I hadn't ever seen him get that emotional. However, I do not regret doing what I did as it allowed for me to dedicate my time to people who valued me more.

Fast forward to now. Since we are in the same degree program, we have all of our classes together now. We went 2 years without seeing each other. Due to the angling of our seats, I catch him turning around to stare at me on a daily basis. Whenever I have to talk to him, he stiffens up, grips items intensely, or looks in the complete opposite direction of me. I overheard one of his friends ask him why he kept staring at me and one of my friends confronted me about it.

Why would a person be this stuck on somebody else after so long? I genuinely don't consider myself all that memorable so I'm confused by it. There is a part of me that also feels bad for seeing somebody in such distress (please call me out for being wrong on this). What I mainly want is to not have such an awkward classroom environment, since I still have to interact with him in small doses.