My subjects keep rebelling, what to do?
The easiest thing is to take a look into the chapter How to care about human relations. If you didn't find a sufficient answer there, you would consider amount of lands you possess. If you have more than two duchies every next gives -10 to relations with vassals. Also having a kingdom may be irritating for your subordinates.
The most often a desire for the crown of particular kingdom have its citizens (especially if you're "stranger"). Particularly frequently those are dukes having lands in the troublesome kingdom. You can solve it by titles distribution: to persons who want them (it'll calm them, but may be problematic in the future) or to someone completely else (hatred will concentrate on him).