Next time you find yourself in the woods, pay special attention to the branches of trees. Notice how for many trees the branches closer to the ground are often leafless and dead, while those higher up are leafy and thriving.
It was not always this way. The branches closer to the ground were once alive and full of leaves. As time went on, the trees grew taller and sprouted new branches closer to the sun and less obscured by other trees. These new, higher limbs often reduce the sunlight reaching the lower branches. Trees allocate resource efficiently and this often means the withering of lower branches.
Friendships, especially old ones strained by distance, can often behave like this. As you grow and branch out, sometimes older friendships are no longer nourishing and less energy is devoted to them.
Obviously we are not trees. Humans are unique in that we can think about the things happening to us and worry in ways a tree never would. It can cause great distress to feel like a cherished friendship is ending and people can believe that this reflects poorly on their character.
We are not trees, but we are part of nature and studying trees can help us accept growth and decay as natural and, in some cases, necessary for survival. Perhaps your friendship issues are a negative development, but growing apart is not always something to feel ashamed of. It can be a sign of personal growth.
Be glad we are not trees in that even when you outgrow a friendship, your friends stay with you forever.