There are obvious strains in the relationship between Dean and Sal, and I think, in a way, both of them are struggling to grow up a little.
Sal goes to school and whatnot, and starts to be more insightful about his own life, realizing his own loneliness and the social structure of where he lives.
Dean has that weird almost-OD when he sees everything he's done in his life and all those things.
BUT
They still leave YET AGAIN!
And that is so infuriating...
Btw, those descriptions of Dean's thumb were SO unnecessary.
Okay, just kidding, maybe they are essential to seeing that Dean is not immortal. He can be injured. In the beginning, he's portrayed as this wonderful, god-like being, but we're starting to see that he can be hurt.
And I think he realizes that himself. As we said, these guys are terrified of death, and Dean sees that he can die. Perhaps that's why they left on their trip. You know, carpe diem, experience life before they die from infection or an accident or something misfortunate...