metamorphosis. yes there's a man transforming into a literal monster and yes it's about sam's descent into demon blood freak monsterhood but to me, above all, it's the episode in which dean's controlling codependency takes a monstrous turn.

he's petty and he's jealous and he's violent and he hits sam for lying and he hits sam again for talking back and he's trashing the motel room and he's the image of an abuser. dean is threatening to leave sam and to hunt sam and they both know he's not going to do either (i can't escape you, you can't escape me). there's dark fury and despair and panic in dean's eyes because sam (in tears, bleeding, taking it) is becoming more than he can control.

sam says "this blood, it's not in you the way it's in me" and dean can't fucking stand it. he immediately tries to write himself into this, sam's not doing this alone. that's his brother, his blood, how dare he go there. their blood not being the same is the gravest, most personal insult to dean.

sam says "i'm not doing it for you" and "this is my choice" and dean cannot STAND IT. sam still craves autonomy and dean STILL. CAN'T. STAND. IT.