A/N - So the word for this is purge, and I think I may have strayed a bit too much from the meaning. 100 words is too short to "rid (someone) of an unwanted feeling, memory, or condition, typically giving a sense of cathartic release" (taken from dictionary), so it morphed into something slightly off-kilter. Oh well :)
Dean knew that Cas didn't think he'd noticed, but he had.
Whenever the angel stumbled in, exhaustion weighing on each step, Dean's heart throbbed painfully. Seeing his angel in pain made Dean hurt too.
'Cas.'
The angel's tired blue eyes snapped up to meet his.
'Cas, this war isn't your fault. There's always gonna be casualties, you can't stop that. All you can do is keep going and stop blaming yourself.'
Cas smiled gently, and kissed Dean's cheek softly. 'Thank you, Dean. I won't.'
The look in his cerulean eyes told Dean he was lying.
But a wingbeat later, he was gone.