It's human to expect good things to last forever, and bad things to end instantly. If things don't end up like that, we think we have reasons to get upset och depressed.
The fact is that it's an absolutely inevitable turn of events. Everything that has a beginning has an end. Regardless a friendship turns sour today or after ten years, it's bound to happen. No matter how long you hold on, eventually the person will die from old age, there's nothing you can do about it.
Be happy that you got to experience the bonding and the getting to know each other, aswell as the actual period of friendship and respect, and even got to go through the separating phase. What more can you ask than to be a part of the whole cycle?
Everything conditioned is unsatisfactory.