Dean was his mother, his father, his brother, and his best friend, all rolled into one, plus a little guardian angel. Sam never thanked him for it, no, Dean would be mortified, would brush it off, hated that sentimental "chick flick" stuff, but he (Sam) knew he wouldn't be half the person he was today without the eldest Winchester brother.
So when Dean died in his place, there was no grief, only a numb sort of blankness, an aimlessness, the repeating of 'NoDeanNoDeanNoDean.' He was nothing without Dean, or nothing good, anyway; he knew it, Bobby knew it, hell even his dad and that yellow-eyed bastard knew it. And so, for a time, he was lost.
Then Ruby showed up, with her goals, and her anger, and all those dark emotions he knew he should feel upon the death of his brother (his everything). That was when he took up hunting again, and hunting was so very intertwined with everything that Dean was, everything Dean had ever been, that it was like a drug to his system, his Dean fix, once he started, he couldn't stop.
He killed a lot that month. More importantly, he liked it. Because Sam+Dean=Winchesters. But, as everyone except his older brother seemed to realize, Winchesters-Dean=Nothing much.