Key point here is one simple statement.
He's not enjoying the racing anymore!
And unless we're talking about one of those top secret, hush hush government jobs that you 'don't walk away from' for fear of being gassed as you're packing your suitcase and waking up in a replica of your room on a strange island policed by vicious weather balloons you see all the time on telly I think it's perfectly reasonable for him to say, enough.
I think he'll still race in something somewhere, who knows what interesting things will come from this?
Plus, Rossi on the Honda, a bit more Lorenzo / Rossi rivalry, who wouldn't want to see that,

I find it odd people take umbrage with Stoner, he's not flamboyant and entertaining in the pits and interviews like Rossi or as plain down to earth funny as Cal and he probably can't make a cup of tea to rival Guy Martins, (ok, not GP I know), but he's a bloody good racer and he takes it seriously, that's why he gets grumpy and @@@@ed off when things aren't right, after all, the race is what GP's all about, not whether he looks lik a fun bloke you'd invite to your BBQ, you can turn off once they've passed the chequered flag if you want.
(One reason I love Sky+, I record it then zip through the 1st half hour of pit lane waffle to the race)