I remember everything about this fic except the title which would let me find it in my history...

It's a gen Bobby POV, with a dual storyline, in Season 5 and Stanford-era (leading up to Bobby running John off with a shotgun). Both times Dean has been affected by a creature (I think it begins with a 'd', but I'm not sure) which makes him act feral, wild, unapproachable, and also slowly lose control of his body. The curse (for lack of a better word) is standard to the creature and eventually leads to death. The only way to lift the affliction is by inducing very strong emotion in the victim. In the past storyline, John induced negative emotion; in the present storyline, Sam goes for positive emotions. Hopefully that's enough for someone to recognize it? There's also a car show...

(Hope I did the tags right; I agonized over 'hurt' vs. 'sick'.)