Is there a reason why none of my friends feel permanent?

It seems like I can’t keep friends for more than 5 or 6 years. I’m the kind of person who regularly grows, changes, and tries to improve myself, but I still stay the same general type of person. I’m very social and outgoing. I don’t understand why I struggle to keep friends, because I get along with nearly everyone I meet, and I do my best to be consistent and considerate. Even the friends I currently have, I just don’t feel like I’ll have them forever. They feel temporary and I can’t grasp why. Any ideas?