Some of the most common shoulds are: I should be able to find a quick solution to every problem; I should always be at peak efficiency; I should never be tired or get sick; I should be totally self-reliant; I should never make mistakes; I should always be happy and serene, even during hardships; and, I should know, understand, and foresee everything.
People are well aware these are impossible to stand up to. These are not "should" that are common to think among people, and phrasing it like this is dishonest. This isn't the way people think.
It's never "I should never make mistakes" it's "I made a mistake, I fucked up, and I am going to be punished for it, I shouldn't have made that mistake". It's never "I should always be happy" It's "I am having a shit time, and taking it out on my friends, hurting them, I shouldn't be doing that, I should try to not let my frustrations out in public". It's never "I should always be reliable" it's, "I wasn't reliable and now my reliant member of my family is under harm, under stress".
We don't seek to be perfect because of some unrealistic expectation of ourselves, we seek to be perfect because not being so is a bad thing.
And you can't say that we should just stop feeling like the above, or face those things, because it's never ok to say "it's OK that I failed and hurt someone".
The lesson here is not that we should accept that we make mistakes, but instead to accept that the past exists, and that what we did is done, to dwell on it and let it determine our future is what we shouldn't do. This isn't a symptom of expectation, its a symptom of being able to let go of failure, or to not let our failures define us.
We should aspire to be great, and expect greatness of others. However, part of that greatness is to understand and deal with failure to meet those expectations well.