Like Putter pointed out, it's from comparing ourselves to other who represent what we wish to achieve. As such, we put that image on a pedestal, and the idea of ever achieving it 100% is subconsciously a foreign concept. What I mean is that I highly doubt anyone ever looks in the mirror, objectively assesses their physique and goes "yep, I'm there" -lol
I would always be reminded that I truly had no concept of what I looked like when I would compete. I'd be backstage, lined up with the guys in my weight class, and I'd be eying the other guys thinking "holy crap that guy's huge", "damn he's shredded!", "sh-t I'm coming in at the bottom of this one" , and some of the most negative crap you could imagine. Of course then after prejudging someone would show me a video and it would blow my mind seeing myself amongst these other 'huge' and 'shredded' competitors, and always finding myself being placed in the middle of the lineup.
Even now, still recovering my my shoulder surgery, I see things very differently due to 20 years of pursuing an ideal. Yesterday I had to get fitted for a tuxedo for my brother's wedding in August. The last time I got fitted for a suit, the size jacket and pants were quite different than they are now (I haven't returned to my previous size yet, and I'm fully aware of that fact). Still, it certainly didn't prevent the saleswoman from freaking out over the size of my arms when my buddy and I both walked in (and he's a very large framed dude).
While most people would have been happy, it actually depressed me a bit as I argued that I was in no way impressive looking, and my friend who has known me since I was 4 rolled his eyes at how distorted my view of myself has become since hitting the iron.