I've had many stumbles in interpersonal interaction throughout my life, but one trend I have observed among the people I've met is that they seldom stay the same. Their behavior, opinions, and interests are always susceptible to change over the passage of time. You may think you've gotten to know someone, but the longer you know them, the less you recognize them as whom they used to be.
There was one artist I once followed on DeviantArt (no names will be said) who drew a lot of dinosaurs and other prehistoric creatures. His main project was a webcomic with a futuristic setting and a rich bestiary of fantasy creatures he'd made up himself, but it was his paleo-art that appealed to me the most. In fact some of his created species' designs resembled prehistoric creatures which I also thought was really cool. However, as the years went by this artist not only stopped drawing paleo-art and shifted attention to his webcomic, he even told me that he'd grown bored of dinosaurs since he couldn't take as many creative liberties as he could with wholly fictional animals. He then started excising the more dinosaur-like creature designs from his gallery, or alternately reducing their dinosaur elements. All that is within his right as an artist, and it's not my place to tell him what he can or can't draw. It's just that his interests and my own have grown way out of sync since I first watched him.
And he is not the only example I've seen of an individual evolving into someone very different from the one I first met. If anything, it's probably the most painless one I can cite off the top of my head.
This has to be an inevitable consequence of humanity's dynamic thought processes. Of course people are going to change once they acquire new experiences and insights, or grow weary of the old patterns. I myself am like this. I probably wouldn't recognize the guy I was ten years ago, or even six years ago. On the other hand, this same human propensity for change has been difficult for me to deal with since it makes the people I trust less predictable than I'd like. Especially when that change has drifted in a more toxic direction. Honestly, there are few experiences more traumatic than a close friend who turns into an enemy.
Human nature can be so uncooperative.