Well, RedRooster explained the very essence of Feudalism: I cannot rule all my lands, I'll appoint viscounts, counts and dukes to ast as governers.
After many years, the governors don't recognise you anymore as their superior. Rather, they claim the lands you granted them are theirs and they have private armies to match your formerly Statal army.
So, the State is reduced to one family, which becomes private, not public anymore, The concept of State disappears, since every dynasty is a State in itself, with lands, powers and prerogatives in justice, army and legislation.
From the XIIth to the XVth Century, many dynasties get to gather the kingdoms together again and to develop a new form of State. The Capetians in France achieved that.
But every kingdom run different. While France centralised, England retained its particular centralised Feudalism until the War of the Roses, which turned England upside down and prepared the stage for the Tudor's nearly absolute monarchy. Meanwhile, Germany desintegrated totally while the different Emperors tried to get the Princes' support by granting them privileges and public power, and Castile, once quite focused on the figure of the King and the Cities, gave up many "mercies" to the nobility after the Civil War between Peter the Cruel and Henry of Trastámara. Hungary was almost a honorary kingdom until Matyás Corvinus raised a private mercenary army, Poland became an elective monarchy, and Aragon was following its example, had it not been by the new dynasty with a centralising policy, which was source of conflict and civil war.
Every example is different, but all these processes answer to the same concepts: the State is no more, dynasties fight for control. Either to recover royal authority, either to keep the royal authority low and our of your business.
Does the King want more support? Grant privileges, loose authority. Does the king want power? Try to earn the support of others than the nobles. The clergy or the towns. Or foreign countires. Hire mercenaries. Expensive? Improve your administration. Create a Royal Chancery. Ask for loans. With a big army, your noblemen won't stand against you. But then you'll have to pay them and risk to live the fate of Matyás Corvinus: after his death, his Black Army ravaged all of Hungary.
And, of course, any attack on the nobility should lower your vassals' oppinion of you. I don't want to see CK1 ridiculous wars of "the count of Sundgau is rebelling against the Emperor, The Emperor is gathering 1.000.000 men to assault Sundgau and the noblemen agree". That makes no sense at all, and all rebellions were like these.
CK1 was unable to represent something like the Barons' Revolt (either John Lackland's or Simon de Montfort's). Let's hope CK2 can.
By the way, Spurius' ideas are very, very interesting.