I've been on both sides of this, to some extent. I had a very good friend from WFU whom I've essentially lost contact with. I stood in his wedding but shortly after he got married it became difficult to get a response from him. I chalk it up mostly to personality and life. I miss him, but no hard feelings. One other thing that I think has contributed in my personal case is that I didn't pursue jobs or a life where a lot of other people did; kind of went my own away after college. Other groups of friends moved to the same cities and stuff like that. I think that solidified friendships into adulthood in a way that I missed out on by moving away. Again, no hard feelings, but just a tough acknowledgement of the realities of life.