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!

[Ovidiu]

Now, if he acts friendly when seeing you in person, why don't you ask him, directly, face to face, about his FB attitude?!?
Maybe, just maybe, it's been a mistake!...
Just ask him!

[Pictures]

Guys are jerks that's my conclusion about them. But about the Facebook thing, I had a coworker who seem friendly and chatty with me at work never requested my friend request until one day I asked jokingly saying "yeah you don't want to be my friend" but I realize I look different from real life to pictures. Nobody notice me

[Jm]

Thanks both for the advice. The thing is, we are ok in class. But we haven't talked a whole lot. He always comes to class a little later than I do and he always leaves right away after class is done. I agree that guys are confusing but I still do not get what I did that would set him off not wanting to add me on Facebook? Like I know some people have some sort of rule not adding people they barely know, but Fb is also to connect with new people?

[matt m-h]

Some guys are just paranoid about who they have on their facebook pages. For example, I have two friends who I am very close to but they refuse to add me as a friend on their facebook pages. It might be the same thing with this guy. Plus, facebook isn't the be all and end all when it comes to who your real friends are.