I think he's just lost the desire, and it's not hard to see why.
He's won both European club competitions as well as the World Cup and European Championships, he's only really missing a league title in terms of team achievements so what else has he really got to achieve? He's been at the very top, lost a lot of what made him so dangerous and with not a lot else left ahead of him why go to the effort of re-inventing yourself at great effort? You can just sit back on your millions, see out your massive contract, have a little run around whenver required and wait for the days to show off your winners medals to the grandchildren.
Perhaps things would've gone differently if he'd taken a step down from Liverpool rather than going to Chelsea, perhaps that could've given him a 2nd wind like Villa at Atletico after Barca rather than going to a more expectant club with a massive fee over your head when your game has already largely gone to shit.