It's a careful tightrope presidential candidates must walk. As I noted earlier in this post about Michelle Obama, voters want a candidate who's willing to get out there and actively seeking their support (and who presumably, feels grateful to have it) -- not one who acts like he's making a "sacrifice" to campaign for the most powerful job in the world.
On the other hand, an air of neediness and desperation to hold the office is never attractive either -- a fact that partly explains both Gov. George W. Bush's defeat of Al Gore, and the unattractiveness of both Clintons' obvious obsession with the attainment (and maintenance) of political power.
Could this "fire in the belly" meme simply be a symptom of the way Obama's behavior is currently being construed -- that is, the same behavior that was so darned cool and appealing when he was on top is being interpreted as problematic now that he's hit a political rough patch?