Not physical strength, strength in any field or skill.
I could understand an instinctual drive to appreciate strength, but besides that? Anything?
Everyone is fighting some kind of demon.
If you fight demons in the outer world, you are applying yourself 100% to an external goal, and you gain from a win. The loser yields something, and the win strengthens you.
If you fight demons in the inner world, OTOH, you split yourself in half, and a win in one area is simultaneously a loss in another. With yourself as the enemy, it’s difficult to gain from a win; a win weakens you as much as it strengthens you.
I’m not saying that people shouldn’t fight their inner demons; after all, you have to start somewhere. But a strong person is perceived as someone who has already conquered his inner demons and has gone on to fight in the outside world where the prizes are bigger and more tangible; whereas the weak person is perceived as somone who is probably still stuck at the earlier stage of trying to master himself before he can take on the world.
Hence the stronger person is seen as more admirable, more in control of himself and his environment, more "mature" in a number of ways.
Just brainstorming here; don’t take it too seriously.