Why does he hate me so much?

[Romi M]

So there's a guy I really used to like--I still like him, and I just wanna be friends with him just like the old days. He used to be really nice to me and he used to flirt with me a lot. However, recently he has become really mean to me; he used to smile genuinely at me, and now he gives me forced smiles. He still addresses me and asks how I am , but he is not as friendly as he used to be. He sometimes helps me out such as trying to find a street address or something like that. If I talk to him, he will continue the conversation, but only if I talk to him first.
I tried adding him on Facebook once but he never added me back. I addressed this with him today and he told me that he has over 50 friend requests. He also said that he hasn't added anyone in over a year and that he isn't adding anymore people. However, on his Facebook profile it says that he just added some guy I know this month! I feel really upset and I don't know why he despises me so much. He does still talk to me, and he does joke around occasionally.
Once I caught him checking out my best guy friend's profile (he doesn't know him), but he could just be Facebook stalking him. It seems like more than a coincidence.
With all my friends, he is really nice to them and he even flirts around with a couple of them. But with me, he throws this attitude and he acts as if he is all high and mighty. I just wanted to be friends with him again and now I feel like he will hate me forever :'( Please help!