Are we over...and why?

I was engaged to my perfect man and with him for six years. We had a really good relationship.He decided to move away last year to college and I stayed at home but am also in college. At christmas we celebrated our 6th anniversary. Everything was wonderful. Then all of a sudden, he stopped ringing and I tried ringing him. He refused to answer my calls. Eventually when he finally did, I asked him what was wrong.He told me that he didnt know, that he was feeling confused about us.I asked him if he wanted to break up and he said he did.I asked him if he still loved me and he said he didnt know.My immediate reaction was “Oh he’s seeing someone else” but after looking deeper into the situation and asking a friend whos going to college with him, I found that he wasn’t. I don’t know what could have happened between christmas and now.He did love me.I dont understand how men can just switch off and decide that, thats it, its over! especially after 6 years.I do know that he was only 15 when he met me and maybe he feels like he’s missing out on something! He is so definate about his decision.Please someone help me understand how he could just switch off like that.