I've had it happen to me, I am sure it's happened to you.
It's a beautiful winter day, the snow on the mountain is gorgeous, you're skiing as well as you ever have, feeling pretty good about yourself. Then your mood changes. A far better skier races by, her form perfect, you notice everything that's wrong with yours. Now you suck, where 30 seconds ago you didn't.
Often people come into Music Room One to listen and leave not able to go back and face their stereo system again. Not until the memory wears off and things are right again.
I think comparisons this dramatic are valuable if you're in the right frame of mind. Perhaps you're curious where you stand next to a reference. Then a comparison helps you gauge your distance to goal.
But generally it's best, if possible, to blow it off and enjoy what you have for its own merits.
Hard to do, but good when you can.