Disclaimer: I don't own Supernatural and never will. No copyright infringement intended.
When Sam's faced with something he can't handle, his first instinct is to shut down. So after Dean disappears – Dies? – And he melts down in the Impala, he shuts down emotionally. It's a defense mechanism that works and all he can think of is that at least there's no Ruby to fuck him up this time.
So Sam finds himself lost in the fog of grief and loneliness and just drifts along until hitting the dog. He still feels guilty about that, but at least he has a good home now. And Amelia is nice and he thinks he could love her, but it's hard to pretend that he's happy. Because Sam can't be happy when there's no Dean. That's the way it's always worked. So he and Amelia break up and fuck a few more times after that.
He can't help but feel guilty about his actions for the past year when Dean finally returns, which Sam hates. He's felt this way for years and it's never-ending.
When his emotions reboot and start going online again, Sam can only hope that Dean doesn't hate him (again) and will eventually forgive him.
He can't handle life without his brother again.
Author's Note: While I don't like Sam's actions after Dean's disappearance, I understand. He always tends to shut down in times of emotional upheaval. So I decided to explore that in this fic. I hope you enjoyed!