Change is a must for us all. But some can't accept the change. Changes comes in all form, it could be physically and mentally. But for some people they have this ideal of one another and it's hard for them to accept change that happens during the course of our lives.
Change can be difficult, especially for creatures of habit.
However, if a person cannot accept it, they cannot move forward. If you come to this point in a relationship, it's time to go your separate ways.
As for people having an ideal of another person, I think that is another topic altogether. When you hear about or meet a person for the first time, you create this idea of what this person is like. Whether or not they are like that in actuality is a different matter. Sometimes your ideal of them is wrong. When it shatters, the person has not changed. Your perception of them has.