friends or colleagues

I would love to interview this one specific friend. For about a year we were so close. We saw each other almost everyday. He migrated to another city but still we kept in contact. He used to say that nothing would change between us, we would still keep the communication going. But one day he just stopped. Without warning or anything. Though I know why he stopped. But still he could have just told me that he couldnt contact me anymore. I would have respected his decision. He didnt really have to avoid me or anything. Well, I mustered up some courage and talked to him when he visited town. We're okay now but of course things wouldnt be the same. He was kinda demoted from a close friend to just an acquaintance. Anyway, having things alright as it is. I would still want to know "what happened" on his end at that particular time. Disclaimer: we did not have a relationship and no, we weren't going down that road. We were just complete friends who understood each other well.