Thank you 332249 for correcting my mistake
It was unavoidable he supposed.
Hunting with Sammy, at some point he'd have to figure out.
Dean had just been bitten by a werewolf. That gave him less than five minutes to kill himself and keep from becoming one.
So they rushed back to the hotel, and Dean promptly shot himself, past where the bite was to ensure the bad blood left.
After telling Sam not to freak out of course.
Sam begged him not to do it and Dean tried to explain, but he didn't have the time, so he just did it.
He woke up to Sam sobbing over him and sat up saying nothing but "Told you so."
"Dean. What. The. Hell."