"you're not the person I thought you were", "I don't think of you the same way I used to", "Things have changed and I don't feel the same way anymore", "Now that I look at us, I feel that this isn't right".
All of these come from setting expectations too soon in the relationship. I always understood this to be dangerous as it sets up that person up for failure. High expectations that are set too soon in a relationship will always come crashing down as we human's tend to be sloppy and lame in our most comfortable state.
For example, the first few months in a new relationship will always seem like heaven. Everything feels perfect & your partner seems like a godsend. When all guards are down & the true character of the person is exposed - all expectations and presumptions about this "perfect" person come crashing down. This may not always be the case, but it certainly is the situation in many crumbling relationships.
The best way to counter this issue is by understanding that the human character is frail & weak. It's vulnerable to imperfections and cannot sustain a glowing review for long period's of time. When you look at the person, you should always expect that you will be impacted by their fallacies and you should endure {if the fallacy is not abusive, destructive, etc.}
I never like to set high expectations, I do realize that human's are chronically imperfect & that chemistry will not always make things harmonious. But what's most important is that issues can be solved with rationale, love & respect dictate the effort put into the outcome, and change is something that should be taken seriously.