why am I crying?

My boyfriend and I live an hour away from each other, since I go to school. I see him on the weekends, and every time I have to go back home I cry. I mean, I miss him and all but why do I cry? I've never been so emotional when leaving someone before. I understand that he's here and only a phone call away. But I don't know. He also doesn't really tell me he misses me or anything when I am away, but just because that's his personality. I know he loves me. Any tips?

I love him, but I also haven't told him... Because he's very cautious when it comes to relationships and I don't wanna seem needy or clingy. Have yo guys experienced this? I can't stop crying when I'm away from him. This started happening a month ago and we've been together for 9. 

Thank you!