Currently I'm studying anatomy and Greek tragedy--separately, and independently. (To answer your question: Because I'm like that.)
You read the post on Heraclitis, on transformation coming about through war, specifically a fight inside the thing trying to change. Stability is an illusion. An internal opposing faction challenges the status quo until it becomes what it is not. What it is now is, possibly, neutral, and what it becomes is simply different, not necessarily better, or bad. All that is sure is that change is taking place, that the antagonist cannot rest until its efforts are rewarded.
There's an antagonist involved in anatomy, as well. The prime mover in a movement is called the agonist--the one that gets top billing in names of exercises. But almost always there's another muscle involved that serves to slow down the movement, and this is the antagonist. This braking protects the joints from unnecessary and potentially harmful stress. Throw a ball, and your triceps will get your elbow moving. But your bicep--the antagonist--will slow down the extension and protect the elbow from undue impact.
The word antagonist typically has a negative connotation; its definitions include the words opposing, hostile, interferes. But come to the meaning having to do with anatomy, and you find counteracts, which is a little friendlier. And its Greek origins: to struggle against.
Opposition can produce a desired or different result, to the point of making better on the original desired outcome--i.e., I want to throw the ball with all my might, but, thankfully, something's looking out for my elbow. The negative is incorporated into, makes possible, the positive.
I saw this in my former prisoners. They seem determined to use their past mistakes, which live with them always, to live gratefully in the present, even repeating these stories for others in the hope that some good would result. To one man I assigned a line that he had spoken once in an early meeting: "I'll never turn back to my old life." Every time he said it--in rehearsal, in performance--he cried.
About boxing, another subject I like to study, the ancient Greeks said this: "A boxer's victory is gained in blood." Which is saying the same thing, far as I can tell.