Did I miss something?

It was maybe purposely made to be unclear, in order to add to the strangeness of Mark's relationship with du Pont. But I saw most of the movie as Mark yearning to get out from under his brother's shadow. Once Dave came on as the assistant wrestling coach, I think Mark's feelings of inadequacy were brought back to the surface once again. And then once Mark started losing matches at the championships, he just couldn't take it...I think he felt like he had let du Pont down. But also, I think the odd verbal abuse that du Pont attacked him with every now and then; not to mention the physical punishment of wrestling - just took its toll on Mark. Not only was he under a great amount of pressure to win, and succeed; he had to deal with du Pont's crazy need to control (and/or undermine) him pretty much all the time.

Mark never liked DuPont. DuPont was basically annoying him and interrupted his training to the point where Mark started wearing a t shirt that said "leave me alone John" or something like that. He had a great relationship with his brother and wasn't at Foxcatcher at the same time. Mark left prior to Dave coming there. True Dave was more sociable, but Mark was just as good a wrestler and coach. He was the head coach of a D1 program for a decade.

He was set up as the golden boy, the guy who was going to do great things... and then all of a sudden it became clear that it wasn't enough - HE wasn't enough; John felt that he needed Dave to achieve his ambitions, so Mark's feelings of inadequacy bubbled to the surface...

You didn't miss anything; it's never explicitly spelled out, it's just an undertone that's created.




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"Your mother puts license plates in your underwear? How do you sit?!"

I am sure what others write is true but just based on the film the impression I got us that Mark Shultz character was not able to grasp nuances of people's personal interactions. I got the impression that he was either slightly mentally retarded or autistic while his brother Dave was not sharp. Seemed like Dave would guide him where emotional or social complexities would arise.

So, I got that impression that DuPont was trying to tell Dave to distance himself from his brother when he told him "you can achieve anything without Mark". So Dave doesn't get it and wants to please both people in sometimes socially awkward ways. Maybe he wants to do what his brother tells him but is confused by what DuPont tells him as well.