Just an interesting quote, that I have seen in the book The Genius Prince's Guide to Raising a Nation Out of Debt (Hey, How about Treason):
For better of for worse, people did change. Even a fearless soldier would one day hope to return to his family in one piece. Even a philanthropic philosopher might drink to oblivion over unrealized dreams. Wein was on the same page for this point.
However, change itself was not a bad thing. Because people were susceptible to change, they could adapt to new situations. Once politicians acknowledged a change in a vassal, it meant nothing more than adjusting to the new circumstances and reevaluating how they should deal with the person.
If the vassal wanted money, give them money. If they wanted prestige, grant them prestige. If they missed the place of their birth, station them there. If they wanted a distraction, toss them in the red-light district.
People change. But there is one constant: They will always have desires greater than serving the nation. All you can do is offer an incentive to keep them as satisfied as possible.