Consider in a life or death situation Cas accidentally leaving a handprint on someone else... (maybe Bela or Kelly). Dean feels really awful about it but can't say why.
Well, that's diabolical. Why would you want to thrust Dean deeper into the torture nexus, why?
Also, I like it. The whole episode could revolve around <other person> being poisoned from the wound (i.e. seraphim -> "burning snake"), and the team learns a lot about healing seraphim-inflicted wounds.
In the end, the healing depends on how deep the wound goes.
Luckily, <other person>'s burn isn't too deep. It can be healed by <expert> doing a <fancy ritual>. Dean's in the room when the attempted healing occurs for <other person>, and it heals THEM but reveals that Dean's wound is STILL THERE AFTER ALL THESE YEARS.
It reactivates on his arm in the shape of his scar in a glowy-glowy way, and he learns from <expert> that his particular wound goes SO deep that it can't be healed... And more importantly, it doesn't want to be healed.
Dean's soul is like a grouchy dragon, all twirly-whirly, viney-snakey-wrapped around the deeper levels of it, protecting it.
