A lot of people are afraid of change, it's understandable, we would all rather deal with what is familiar to us. But when People change, things seem to get more complicated. Everyone likes to act as if a person changing is inherently bad. But it isn't. Change is a part of personal growth. Learning to not hate yourself, or learning to put your own happiness over that of people who don't care about you, is a good thing.
That's why it irritates me when I hear someone say they don't talk to someone who they used to be friends with because they "changed" without even knowing the reason why. It's why I cry in my room when people treat me with disdain because I'm not the exact same person with the exact same interests that I had a few years ago.
When you treat someone badly because of a change that they have made in their life, you tell them that growth is not okay. That what you expect of them is more important then their own happiness (and in some cases, their mental health). In my mind, that is a terrible thing to do to a person that you claim to care about.
This often strains relationships, because one person will feel unaccepted by another. Which can especially be hurtful when it is coming from a person that you care about deeply. The way to fix that relationship is not to tell them that they need to go back to how they were, or to be more how you want them to be. If there has actually been a negative change in the person, you should talk with them about it, and help them work through it. Not simply tell them that they're wrong and you're right.
Everyone has had their own thoughts and experiences. And, everyone reacts to them in a way that is unique to them. Telling someone "you've changed" is pointless. We all have, it's part of what makes us who we are. And pressuring someone to be different is not going to change that. This is something that all of us need to accept if we want to have a healthy and long lasting relationship with the people that surround us.
By accepting a person as they are in the moment, and not the person you think they they should be, we can actually learn to work through problems that may arise, instead of leaving everyone at a standstill in a state of constant tension.