To clarify: an internal arc is a character's emotional and/or intellectual journey from one state of mind to another by choice; it will be accompanied by an epiphany or a-ha moment. For example, in The Lion King (Walt Disney's Hamlet), Simba's face off with Scar is the climax of the external arc. Simba's acceptance of his role is the internal arc--as it is in Hamlet when the eponymous character states, "We defy augury. There’s a special providence in the fall of a sparrow. If it be now, ’tis not to come. If it be not to come, it will be now. If it be not now, yet it will come—the readiness is all." Hamlet has made peace with himself and the weirdness of life: he has resolved his internal questions.
Why Dean is the necessary keeper of the internal arc:
(1) Sam's internal arc would remove him from the storyline.
Sam's internal arc occurred before the show begun, climaxing when he left the "family business" for college. The series starts when his father disappears, his brother shows up, and his fiancée dies in a macabre fashion. But in his heart of hearts--as revealed in "Dark Side of the Moon," the episode where Dean and Sam visit heaven--the emotional journey that enabled Sam to leave hunting remains incredibly meaningful to him, defining him as a person. Leaving WAS the fulfillment of his inner desire to be his own man.
To follow Sam's internal struggle to its natural conclusion would be to remove Sam from the family business permanently (or produce a Sam so bitter and resentful that nobody would much like him)--and whoops, there goes the show!
Consequently, much of what Sam endures is external--being tempted by demon blood, being stalked by Lucifer, even losing his soul. All these arcs work because they resonate at an emotional level; however, they are external struggles: they were done to Sam (passive voice).
Don't get me wrong: part of the show's attraction is watching HOW Sam deals with these things. Still, Dean runs the internal arc because he produces from within himself an emotional journey of guilt and doubt.
(2) Sam is better at cognitive dissonance than Dean.
Overeducated people in general are better at cognitive dissonance than less educated people.
Note: I did not say, "over" and "less" intelligent. Smarts, commonsense, discernment, the ability to comprehend the world: all these things have nothing to do with education. Education, in general, does three things: (1) provides people with usable skill sets (ability to read, ability to add); (2) creates habits of discipline; (3) provides exposure to multiple ideas. (#4: In today's world, it enables people to compete in the marketplace.)
#3 has an interesting pay-off: more exposure leads to an increased ability to balance disparate ideas at the same time. Aging does the same thing, which is why as people grow older, they tend (on average) to get less dogmatic/black & white in their thinking.
That is, as people get older or become exposed to more ideas, they become more comfortable with hypocrisy, a good thing since such cognitive dissonance (I totally believe in freedom from government tyranny--I still pay my taxes) allows the human race to survive.
Terrorists and saints find the disparity between ideals and reality less comfortable. Legal student Sam is comfortable (in the earlier seasons at least) with contradiction--all the horrors that he has seen do not negate, for him, the possibility of angels. This would be awe-inspiring if it was based on some large theological view of the universe. In Sam's case (and I think this is quite realistic), it is based on the educated man compartmentalizing his ideas because he knows that many ideas do get compartmentalize, simply by necessity.
Dean, on the other hand, is never comfortable with contradictions. His education is the education of experience, not the education of ideas. When experience contradicts reality, he suffers confusion, disillusionment, and angst--all elements of an emotional/internal arc.
(3) Dean is a non-boring idealist.
In many ways, Dean is an idealist. Idealists invite instant arcs, being primed--so to speak--for worries followed by large personal sacrifices.
|"What do you think Death does to people who lie to his face?"|
Dean escapes the worst aspects of idealism: he is not a terrorist and eschews being a saint. He remains interesting (especially) since his entire personality is underscored by hard-headed realism.
While Dean doesn't accept the contradictions neither does he avoid them. Sam is the guy who either tries to figure things out (make the contradictions square with each other) or deals with them separately (this is what I can do today); Dean is the guy who agrees that the Apocalypse is coming, it can't be stopped, earth is doomed, and we should all give up--then goes to the field to be with his brother anyway. Dean is stoicism squared.
To put it another way, Dean rejects cognitive dissonance because he won't try to believe two seemingly contradictory things at once--instead, he decides that trying to make stuff square is wishful thinking, right before he goes straight at the problem. And since he actually is a Saint (he just doesn't know it), the solution that nobody ever even thought of occurs.