Why did Gen. Petraeus take the job in Afghanistan?

I'm still wondering why Gen. Petraeus took the job in Afghanistan.

Reasons not to (in no order):
While there is no reduction in rank or pay, it is a less prestigious position then head of CENTCOM and nominally a demotion since he would (in theory) report to whoever replaces him at CENTCOM.
A positive outcome in Afghanistan isn't a certainty and failure would have a negative impact on future ambitions, either Chairman Joint Chiefs or his rumored presidential ambitions.
He couldn't really be forced to take the job either, since at that level 4 star officers (General or Admiral) have a fair amount of say as to what their next job might be.

Some reasons he might have (in no order):
A promise from President Obama, that the President would endorse/recommend him for Chairman of the Joint Chiefs when Admiral Mullen retires
An agreement that Gen. Petraeus could run things in Afghanistan as he saw fit (within reason), meaning an relaxation in the Rules of Engagement, and a reasonable request of additional troops if required (likely 50,000 or less) and no meddling by the military and civilian bureaucrats in DC.
Future political ambitions, if he is successful in Afghanistan, it will significantly increase his chances if he chooses to run for President. Something along the lines of "I was able to succeed where the President and his handpicked military and civilian people failed"
Doing it because he thinks it is the right thing to do for the country, the military and men and women serving there. (Have I become so cynical that I have trouble believing there are still people out there who'll do the right thing even if it might cost them future potential gain)

thing to do - His President asked for his help, and he gave it.

When the CIC "asks" a senior officer to take a specific assignment, it is understood in the military that it is an implied order. Failure to accept the assignment would have been the end of his career.

If Gen Petraeus had said to the President, "No thanks, I'll retire instead" it would have made the President look bad.

Besides rank wise he is at as high as he will ever go, any increases in pay will be based on years of service so he'll get no major increases in pay. (I just went and checked and he has 36yrs of service so he is making $17786 a month base pay, if he makes 38 years of service he will make $18675 a month base pay.) He MAY end up in a more prestigious post, but he is also getting fairly close to mandatory retirement and there isn't really enough time to fix Afghanistan and still have time left before retirement to get the Chief of Staff US Army or Chairman of the Joint Chiefs of Staff position and serve out a complete term in either of those spots.

He probably is doing it because he believes it is the right thing to do.

I think the idiot actually believes that the war can be "won" and that he'd be a superstar general in doing the impossible. The man should be in an asylum.

It's not as though he's a lucky job-seeker weighing his options. He either follows the commander-in-chiefs orders or he resigns.

I'm mostly arguing over semantics because I believe that the word "resigning" has negative implications that retiring doesn't.