Why guys seem friendly at first but then act like jerks?

[E]

I started liking a guy this quarter. He seemed nice, smart, funny and cool. We had a midterm a few weeks ago and I pretty much planned the study group. He joined and he would either sit next to me or across from me. There were a few instances where I caught him looking in my direction but I could be wrong. One day, my spot up in the front of the class was taken and so I decided to sit next to him. I was just being friendly and all and we had a good conversation. I thought I could get to know him so I added him on Facebook a couple days later. A few days later I checked if my request was sent and it turned out that he rejected my request and it went back to "add friend". What gives? Why are guys hot then cold the next? It makes me think he's a jerk for not wanting to get to know each other and maybe be friends. He added a couple ppl in class btw. He's all for social media so I'm not sure what set him off? I still think about him sadly Would appreciate some advice. Thanks!