Why does my crush act hot and cold with me?

He goes to my church. We used to sit together and he would stare at me and make conversation. He once grabbed my hair and tapped his hand on my knee. He would usually always say hi and goodbye, and he was always really nice and attentive, he would sometimes even make it seem like he was following me around. He would do me favors. But yesterday I was with a friend (girl) and he didn't want to get near me. He said hi from far away and he didn't say bye, and he simply just acted really cold. He is 21 and I am a 15 year old girl (and yes, I know that I CANNOT pursue him because it's illegal, but he looks 17. I just want to be friends and I enjoy his company.) I once asked him what his age was, since he knows mine, and he didn't want to tell me. He told me to keep it at 18, since I had tried to guess, but he seemed playful. He sometimes teases me, too, and he acts sarcastic. I want to understand why he acts this way. PLEASE GIVE HONEST AND SERIOUS THOUGHTS.