See also
Sam Winchester

What drives him to turn at the end?

*jealousy at how much Dean relies on and trusts Castiel
**feeling like Castiel is taking his place, since it’s been just the two of them for so long, but Dean seems to prefer Castiel’s company now
*ultimate rebellion against John (and Dean)
**Sam was insecure about his place in the family in the early seasons–maybe that’s never really gone away
*can’t fight destiny
**This is a big one: feeling like there’s no point in fighting destiny, feeling despair because of his destiny, etc.
*he has bad dreams every night after they first get the cup, and he thinks this means the cup doesn’t like him/is rejecting him (in reality it’s Lilith messing with his head? or Lorcan messing with his head, even? No, Lilith.)
*I don’ think he cares about Ruby enough for anything happening to her to make a big difference to him, but I could be wrong about that. He’s not the same sensitive Sam from season 1, though.