Crusader Kings II is somehow more a RPG than the strategy, because the human relations are the key to the victory. From what monarch you are depends almost all shape of the game. If you have to handle the strong opposition, there'll be internal wars, intrigues and murders. Sovereign who can count on subject's support, may manage his estates freely and think about expansion, not only diplomatic one, but also a military (the more support, the more army he get from the vassals). Every character has a window with his opinion among the vassals nearby the portrait.
And as far as providing the peace in the small countries isn't difficult, it grows to the level of challenge with the bigger empires. On character's opinion/authority affects many various factors from which every has meaning. There is no secret in fact, that it's much easier to lose support than to gain it, so read carefully all statements and make choices carefully. Most of them decrease trust of your vassals, but wisely played may assure you a success.