There's this popular quote by Mark Twain -- "Only my tailor acts sensibly, he takes my measurement anew every time we meet; others just keep holding on to their old measurements of me and expect them to fit me."
I seen people hold on to their secondary school measurements of their friends. I hear them say -- "Bunmi is so just not up to my level. Throughout secondary school she was always looking up to me and wasn't half as smart as I was. This amazing feat she has just achieved is definitely by luck." Here is someone trying to judge another by measurements she took over 15 years ago, as if people don't change.
For as long as I can remember, every year my life changes drastically. There has never been any year in my life that as left me close to what I was at the start of that year. I don't even have a static opinion of myself. And it's the main reason I don't care about other people's opinion of me. If I can't be right about myself for long, why should I expect someone else to be right about me?
It is almost a lie to claim to know anyone perfectly. Because even in the very rare case that you actually do know them perfectly, you can be sure that it won't be for long. They will soon change. People change, either deliberately or indeliberately.
In the dealing with the people close to you, you have to give them some room. Don't claim to know everything they will do or can do, or you are setting yourself up for a big disappointment. Don't keep holding on to the childhood memories you have of them, they are no longer the same persons. Don't even attempt to know all about them, you can't. And don't ever say they can't change. People change. Always.