Truly everything else in our lives we'll happily accept our personal bests as great, and more importantly, we're realistic about them. We appreciate that while we can no doubt always do better or try harder that there are fair limits to how hard we can try.
I remember trying to get into medical school. Boy I spent a lot of time in the library and when I didn't get in to medical school on my first try, following my rejection I didn't wish I'd truly eliminated every last ounce of my social life to study - I was disappointed, but I was sure that I had done my best knowing full well that at the expense of real life no doubt I could have done more.
Bests aren't perfection. Bests are affected by real life. Bests need flexibility. And bests change depending on circumstances.
Those folks who head into weight management thinking their bests need to be perfect, that they need to always be on? No doubt in my mind for the vast majority it's just a matter of time before they've frustrated and disappointed themselves often or badly enough that they'll quit trying altogether.
All or nothing almost always gets you nothing.