My son wants to lead armies and get glory. I don't let him. He becomes mad, later rebellious because he thinks I don't like him as much as my other son, who already have led armies in war for five years and have become the hero of the nation.
Yep, that's pretty much it. Each character has on the character list an ambition: this could be "get married," "obtain a barony," "become Duke of Gascony," "become chancellor," or even more precisely, "kill Robert II, Duke of Normandy." If you want to keep your people happy, you should look at their list of ambitions from time to time and grant their wishes if possible. Once one ambition is satisfied, another is given to the character. So it is sort of like personal missions that helps drive the character AI. In Rome, unsatisfied characters joined the "Populist" faction in republics and overall became obnoxious.
I imagine in CK2 that characters with unfulfilled ambitions will be more likely to get dissatisfied and will leave your court, and dissatisfied vassals will be more likely to rebel or plot against you. Satisfied characters will be more likely to support you and watch your back.