Tommen: What about strength?
Tywin: Yes, strength. King Robert was strong; he won the rebellion and crushed the Targaryen dynasty. And he attended three Small Council meetings in seventeen years of ruling, and he spent his time whoring, hunting and drinking until the last two killed him. So, we have a man who starves himself to death, a man who lets his own brother murder him and a man who thinks winning and ruling are the same thing. What do they all lack?
Tommen: (considering, then realizing) ...Being a bird.
Tywin: Yes!
Tommen: Being a paralyzed bird with a touch of autism, and voluntarily disqualifying yourself from being a lord, is what makes a good king.
Tywin: Yes! But what does it mean to be a bird? Do you know anything of Jon Snow's heritage? Have you ever sat in a godswood and done literally nothing while Arya Stark leapfrogs from twenty feet in the air to defeat an ancient evil?
Tommen: (dejected) No.
Tywin: (reassuring) No. A good young king who is a bird listens to his advisors, namely another bird but one that is also a tree at the same time. He allows massacres to take place and then smugly accepts his coronation from a treasonous prisoner-of-war.