It's actually the opposite. Most people are too reasonable. They don't know what good looks like, or even if they do, they're willing to accept much less from themselves and those around them. 

There is nuance here of course. Being an unforgiving asshole about everything doesn't count, at least in my book. 

It's a very fine balance of having the humility to always be pushing yourself to improve (and accepting of feedback from others) but also having conviction (based on your good taste and experience) that you know what "excellent" looks like.