Say you had a friend, who was almost your best friend at one stage, and then all of a sudden things happened, people had better things to do, others to talk to, and you just decided not to bother talking to that particular person anymore. At first you were just pissed off, then you missed them, and then you moved on based the fact that you had plenty of other friends. A sad waste of a friendship really, but it happens. Not often, but it does.
The problem is that even though you don't mind that they aren't a part of your life anymore, so many things remind you of them, from songs you hear that they liked, things people say that they would have laughed at, and it just gets frustrating when you can't talk to them and tell them the things you'd like to.
The question is, what to do in such a situation? The answer; nothing. You'll never meet another person like them, but there's no point in dwelling on the past. Compared to the other 6 billion people out there on this funny little green and blue sphere we call Earth, they are insignificant in existence.
