I learned the answer to this in physics class in HS. Teacher said that an instrument of measurement is only accurate to 1/2 of the smallest unit it describes. So... what that means is that once you think you have something accurate, you can move to a finer set of measurements and find inaccuracies there. And the same applies with music performance. No matter how close you get, there is always a way to see that you should have been closer. Are you in tune, sure, I'm only 2 cents sharp (oh, well then, get a better tuner and practice). Do I swing enough? Yea, probably, but you don't swing as much as the drummer, or 1st trumpet, or guitar etc. How about my tone? Volume? Crescendos? Balance? Tempo? Did I put the note the right amount ahead of the beat... or was I supposed to be behind the beat? How about the style? Did I get it right?
The bottom line is that there always be someone ready to tell you how you messed up. And it's a good thing that asshat is there. That way you can quit beating up on yourself and play some music.