There've been conflicting stories about Brosnan's last days.
On the one hand, aside from the obligatory pre-release press junkets, I gather he was unsatisfied with Die Another Day (and rightly so -- that film SUCKED. Easily down there with Moonraker as far as low points in the franchise), and was sick of that version of the character. On the other hand, he may also have been disappointed that he didn't get to go out on a high note...although in fairness, NONE of the guys who've played Bond have, actually.
Connery -- Diamonds Are Forever. (The beginning of the super-cheesy slapsticky era.)
Lazenby -- On Her Majesty's Secret Service. (I actually really like this film, but many hate it. I think they just hate Lazenby, and mostly then because he wasn't Connery and seemed a poor replacement.)
Moore -- A View to a Kill. (Geriatric bond. Brought to you by Centrum Silver.)
Dalton -- Licence to Kill. (A bit dark and slimy as Bond movies go. Not a bad film, but not well received by the public.)
Brosnan -- Die Another Day. (Crap. Almost entirely crap. With the possible exception of Bond when he's been sold out and having to be resourceful on his own. Which ends quickly and then he gets AN INVISIBLE FREAKING CAR.)
Craig -- Quantum of Solace. (Potentially decent story obscured by GODAWFUL editing. Plus, it's a middle chapter of sorts, so it doesn't provide real resolution.)
I can understand Brosnan wanting at least one last outing to "correct" the crap that was Die Another Day, while still hating the film and the direction the franchise had gone.