Castiel is presented as an internally conflicted character. His disobedience and growing mistrust of Heaven is one of the main threads developed throughout season 4. He discusses free will, the value of human life and condition of humanity with other angels or Dean. We see him deliberate and express doubt, finally we see him openly defy his superiors and become a rebel. There are four situations in which he is at crossroads and three times after an intellectual debate on deontological ethics he chooses to follow his liberal beliefs.

Wait. Did I write "after an intellectual debate"?

I almost got tricked into believing that questions concerning ethics and free will were the root of the problem. Castiel's struggle to reevaluate and reconstruct his world view is a good cover story, but still not good enough. If we take a closer look at each of the situations where the angel has to chose we see that Castiel's motivation is very, very mundane and each time Dean is in the focal point.

First situation in which Castiel faces a choice takes place in a small town during Halloween, when a witch plans to rise Samhain. Two angels are sent to wipe out this town along with its inhabitants - 1214 of them, as Uriel accommodatingly reminds. There is a short, heated exchange in which Castiel fights his corner pretty firmly. He seems honestly convinced that destroying the city is not that bad of an idea. Bigger picture. Orders coming from Heaven. God's plan being just. These are his words. What makes him change his mind and allow Sam and Dean to carry out their plan? Dean's provocative cheep joke when he appeals to the angel's ambition and self esteem. That's right. Lives of 1214 people don't matter in the face of bigger picture and just orders from Heaven. What matters is that if Castiel decides to purify the town, Dean will think badly of him.

The next dilemma is much more serious. The angel has to decide whether to interfere with God's plan announced through His prophet. Castiel still believes that what is written cannot be unwritten - or more like shouldn't, because it obviously can. He changes his mind and decides to help Dean prevent the union between Sam and Lilith after Dean resorts to emotional blackmail - something they will both use very often to influence each other. He threatens he won't help Castiel if the angel ever needs him and leaves in a huff. Castiel relents immediately.

There is this shameful episode 4.20 in which Castiel is held captive and probably tortured or brainwashed in Heaven. Even when he refuses to reveal the secret of Lilith's role in releasing Lucifer he focuses on Dean. " I serve heaven, I don't serve man, and I certainly don't serve you." These words are harsh, but they provide an insight into Castiel's perspective. The fate of humanity is somewhere in the background. Ultimately it is all about Dean.

Finally we have the scene in the Green Room in which Dean and Cas have a violent argument. Many ethical issues are addressed, but again Castiel fends off Dean's arguments without any difficulty. Intellectually he is convinced that the Apocalypse should be started - it should not be surprising given that it is written in the Bible. Apocalypse is not a cataclysm unleashed by Satan. It is the proper way to end the world; it is planned and wanted by God. He is also convinced that the Apocalypse will end Dean's torment. "What is so worth saving? I see nothing but pain here. I see inside you. I see your guilt, your anger, confusion. In paradise, all is forgiven. You'll be at peace. Even with Sam." These are not excuses of someone who is too afraid or too powerless to act. Castiel sincerely believes that he's doing the right thing.

Ultimately the angel decides to help Dean and again this decision is forced by emotional blackmail. The older Winchester turns his back at Castiel and threatens not to speak to him any more. Histrionic? Perhaps. Does it work? Oh yes, it does.

This is not how friends talk. Friends present their points, they try to convince each other and understand each other's motivation. Emotional blackmail, huffs, drama, threats, making performances: these are hallmarks of relationships in which there is little space for reason, because emotions take over. I cannot decide whether Dean does it knowingly and I will not try to discuss his motivation here. What matters is that this kind of drama hits Castiel's underbelly. He loses his head because he's fallen for Dean (in every way imaginable).

Castiel decides to die for Dean. The way he acts and speaks makes it clear that he knows exactly what follows. He is worked up tense, yet focused and determined; he acts quickly in a kind of berserker's trance. If he was human I'd say one could feel the adrenaline pumping through his veins. He cuts his forearm unnecessarily deep and draws an unnecessarily large sigil. When Raphael comes after him Cas doesn't seem surprised or afraid. He doesn't know he would be resurrected, still he has no regrets. Let me put this straight: he is ready to die because a guy said he wouldn't like him anymore if he didn't. This is not reasonable. This is not an outcome of changing one's value system after thorough reexamination of priorities. This is a behavior of a teenager in love.

One could argue that perhaps Castiel made all these choices having humanity in his mind and all his arguments with Dean so emotional and heated because a philosophical debate would be boring. In my opinion there is enough "philosophical debate" included in the show to prove that in fact Dean never actually convinced Castiel, at least in season 4.

If anyone doubts that it was always about Dean let the angel speak for himself:

"And I did it, all of it, for you". "You" is both singular and plural. Castiel could mean humanity, all people in general. He would address them with "you" because he is not one of them.

"and you failed" all right, humanity did not fail. Casiel could mean the hunters in general, the Winchesters or the three men present in the room - Sam, Dean and Bobby.

"You and your brother destroyed the world." The Winchester's don't have a brother. Bobby doesn't have a brother. It makes it clear. Everything Castiel does, he does it for Dean.