Losing friends

      Why is it do people you once considered close friends all of a sudden become people who can't stand you or you can't stand them?  What changed in your relationship?  They seem like the same person, so what happened?  Did you just grow apart?  Or is there some underlying cause?  This happens to people all the time.  Now, I'm not talking about big events like they stole your significant other or something like that.  I'm talking about how one small little fight ends up in the end of the friendship.  It's easy to say that it was the straw that broke the camel's back, but I have a deeper insight on why that was the straw that did it.
    When you decide someone is your friend you let them into your circle.  You allow access to your thoughts, desires and want to spend your free time with that person.  Since people are naturally guarded, the longer you know someone the more you let them in.  After a while, they become so close that you stop looking at their actions as something independant of you.  You become emotionally involved with that person, which means that their actions now affect you.  For me, this normally takes someone six years to become emotionally involved where it's no longer just something I do, but it affects them because they worry or they care.  Sometimes it happens vice versa but not often.  It isn't that you just got sick of their shit, it's that you started to care so much about them that their problems now drain you.  It brings you down when something harms them, and if they keep doing those things, eventually you have to protect yourself by breaking away.  That is how we lose those we considered close friends.  Up until now, it only hurt one of you, now it hurts you both.