Separating from friends is the worst case scenario. Life sometimes appears to have a specific goal of separating individuals, practically as effortlessly and as fast as it unites individuals. The most common reasons for friendships to fall apart is just because life gets in the way. People move away and sometimes you lose touch with your friends. Many friendships develop simply because you are around that person a lot.
Once you are no longer around them as much as you used to be, friendships can sometimes begin to fade. Other common reasons are if you and your friend have a disagreement.