Why would someone be so persistent?

I met a guy last semester at school. It was his last semester and I have one more to go, this fall. He started talking to me in one of the classes we had together by offering his notes. we exchanged numbers soon after and starting studying togther. We ended up pulling an all nighter once and since that day, he started walking me to my classes and we'd go to lunch at least once a week. The thing is, he had a girlfriend, and still does, most likely.

For one of our finals, we pulled another all nighter. Towards the middle of the night, he pulled his chair next to me and sat so close our legs were touching and we were reading out of the same book. He even placed his hand on my thigh on one occasion when we were joking around. That night, i found out that his friends didn't agree with us hanging out. One of his friends who would stick around "to supervise us" yelled at him when he invited me for drinks after our final. He said its wrong.

I was going to take an exam this summer for my degree, one that he already took and he offered to help me study. He told me he wasn't going to graduation and he actually did and we ran into eachother. He ended up texting me at the end of the day to check my grades and we texted for a few weeks straight and i just stopped replying eventually. he suggested we get drinks one day but never said when.

He ended up sending me a selfie on snapchat a few weeks ago and we have been talking on there ever since. He suggested we finally meet up for those drinks, but he wants to get drinks at school since i still go to lab on some days. He also said he wanted to take a class with me this fall and wants to apply for the same masters program i am, when he had told me during the semester that he didnt' want to.

I am not sure if guys normally keep in contact with all their friends like this or what is going on. i just feel weird about the whole thing because he has a girlfriend. should i think anything of it?
Why would someone be so persistent?
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