There are more than a couple of reasons that something like this can happen. One is that he did not share the same level of emotional attachment. Another which I have experienced with long distance relationships. The distance and time is so far apart that interacting is too painful.
There is an expression "this too shall pass". I know it isn't helpful. There is a rule of thumb that for each year that you are with someone you need to take a month to recover from it. While it might be true, I found that it takes me a year for every year... its really annoying.