It's a fact of life that people change as they move through life. They take in new information, contemplate their goals and dreams, make decisions based on more information- all of these things alter behaviors. The more experiences, the more people change, particularly if they are intent on learning and growing. Many times, this means leaving behind those people and things that no longer fit. When it comes to people, separation is painful, particularly when effective communication has not been employed.
Humans are creatures of routine, and will tolerate the intolerable to avoid change. Change can be scary and painful, as it brings the unknown with it. Sometimes, people begin to move down a path, and then run back to their old habits and routines. It feels safer to be in familiar territory. It's all human nature, and there really isn't anything to judge or condemn. It is what it is.
People tend to feel guilty about the decision to end friendships, particularly when the friends have done nothing to really warrant dissolution of the relationship. Instead of simply communicating about what's really going on, they begin to vilify the person(s) from whom they want to disengage, because it's easier to leave an unlikable person than a friend.
As this new construct begins to take hold, assumptions are easily made, fitting this "villain" into the mold. Issues suddenly arise that were never there before, accusations begin to fly. Before anyone knows it, this beloved friend is now a terrible enemy. Walking away is easy now, because there is justification for leaving. What's left behind is a wake of poisoned hearts and damaged spirits, rather than loving farewells and well-wishes.
How much different would life be if people simply said, "I appreciate the time we've walked together. It is time for me to walk a different path. Thank you for all you have shared with me. I am better for having been here with you" ? What if the receiver of that loving goodbye could say, "I understand. While I will miss you, it is time for you to go your way. You are loved, and I wish you nothing but beauty and peace as we part ways. Thank you for this time together"? And, what if both really meant it?