If you read his 2008 season review you can see that there were more than a few times where circumstances out of his control robber him of big points hauls;
- A driving style that did not suit an understeering Ferrari, especially in quali
(which seems to still be very apparent in the F150 italia)
- Drive thorugh penalty for team failing to prepare correct tyres in monaco
(which snowballed into his accident with sutil)
- Lewis robbing him in canada of a win
- Exhaust failure in france gifting massa the win
- Rediculous we weather calls by his team in silverstone
- European GP engine failure
- Italian GP quali fiasco (by both mclaren and ferrari) leaving him 14th
- Handing massa second place in china
And yet somehow matched a record of 10 fastest laps out of the 18 races.
In my opinion only, i think he was very unhappy at Ferrari and a horrible season only compounded his misery. He then took the very next opportunity to return to Mclaren, but for whatever reason didnt pan out that way.
I think the motivation story thrown around is true to a point. Its not tht he didnt want to go racing, he just didnt want to race for Ferrari anymore.