Sometimes as friends come in and out of our lives, it is an unsettling event. We as humans are so quick to take this as a rejection, fearing that it was something we did or did not do. Why our focus is so much on ourselves that it doesn't allow us to realize that it may have nothing to do with me, and everything to do with what is necessary for our friends personal development. Letting go is hard for us to do.
Even the healthiest people want to keep the things that are familiar, the fear of the unknown is a scary thing. We go through our lives surrounding ourselves with the known quantity. We know what it is and what to do in situations, it is familiar and comfortable.
The same thing seems to hold true in friendships that aren't healthy but they are known to us. It seems easier to keep the relationship than risk loosing the contact. Once again, when you face the fear and walk through what is necessary, once you pass to the other side, it looses it's fear.
When friend's journey takes them away from the friendship, it is best to wish them well and hope that their journey takes them to what comes next for them. By allowing this to happen without fear of loss to yourself, it actually allows you to keep the door open to the possible reentry into your life at a later date.