It's surely not a passion for the game. The Miami Dolphins locker room still has got to be pissed at him because of his actions they are probably going to have the worst record in football this season.
If I were the Dolphins I wouldn't take him back and attempt to trade him. As long as its not another terrible trade like a 2nd round draft pick for AJ Feeley. You can argue that they should take him back, but why would you want a teammate that was selfish enough to retire because he would have been suspended for a year for smoking weed? Tell it to guys like Jason Taylor and Zach Thomas who are probably highly angry with Ricky.