The sources are particularly lousy for this: the wretched Appian on the one hand, and on the other Plutarch, more interested in the morbid details of his wasting disease than any politics. Scullard and Keaveney both argued that Sulla simply got tired and decided that he'd done enough to ensure a return to Republican form. Sulla retired to Cumae, into a Campania that he had essentially remade from the ground up in the aftermath of the ravages of the Social War. It was a pleasant place, to be sure, and I don't think there's anything wrong with believing that Sulla went there purely because he wanted to relax and indulge in the proclivities which he seemed to cultivate.

But I don't think that's right. I don't think Sulla was "done," and in fact the actions of Sulla in some ways provided a rough draft for how the principes would elevate themselves above the machinery of the Republic later, starting with his protege Pompey. Striking coins with his image while he still lived (a no-no in the mos maiorum), not to mention the giant equestrian statue of himself in the forum, both suggest Sulla had grand designs. He left off the dictatorship in 81 and held the consulship in 80, probably thinking he was secure enough in his position to do away with the more odious form of authority. His old rival Marius had held consecutive consulships in the past, and Sulla might have had something like that in mind in lieu of the dictatorship. If so, it never came to pass. I think the old madman was probably gently ushered out by the younger Sullani, Pompey in particular, and convinced to let others have a turn. He could have fought on, and there were plenty of Sullan veterans in Italia that he could have stirred up, but he seems to have just simply got too tired. I don't like modern claims that he was "forced out" or the like. Nobody ever forced Cornelius Sulla to do anything. It was most likely a combination of satisfaction with his life's work, fatigue and sickness, and the gentle, careful, but insistent advice of his young proteges.

Check out Jenkins' little article, "Sulla's Retirement," 1994. See also, for a modern and interesting look at Sulla, Santangelo's Sulla, the Elites, and the Empire Brill 2007.