I think part of him might still be feeling guilty over his decision to play TMC in 08 and then burned out against LaLo in a 5 setter when he was expected to win.
Why would he feel guilty of that? Was he supposed to skip TMC?
It has nothing to do with guilt though, he will just never quit until the point of no return. He played AO 2010 with a bad wrist, RG this year with a taped knee, took shots before every match to make his wrist pain bearable during this USO... it's just who he is, no matter how many injuries he will keep fighting and playing through them.
The question here is why the hell the Argentine captain is allowing him to play? Surely any responsible captain would put his players' health first - although, let's be honest, with Nalby out, Delpo winning his two singles is the only chance Argentina have here as Monaco will almost surely lose to both Berdych and Stepanek.
Someone in the Argentine team should just step up and forbid him from playing.