the diaz brothers only fight for money, more specifically they want big paydays....I agree with gsp's recent interview where he said nick waited a long time to come back and left potentially alot of money on the table because of that and hoped that with his recent lawler fight he'd come back healthy, in good shape and win.
Both nick and nate have never been comfortable being interviewed or having camera's or the public staring at them.....they also may suffer from anxiety and have a difficult time dealing with being in the spotlight, that's always been the personality for both brothers, you get camera shy start stuttering, can't put your thoughts into sentences to answer the questions.
One could also say the regular and long term use of marijuana and years of fighting and taking punches to throw punches tactic eventually caught up to them as well.
I say it's probably a combination of all those things. Also nick might of given up at the end of the fight because he just didn't have it in him to continue, maybe physically he was capable but mentally just had no desire to get up and keep fighting...not every fighter can come back from a long lay off and win, I give diaz credit for accepting the fight even with the outcome that happened.