It was a nervous tick, and one he wasn’t particularly proud of.
Dean chewed toothpicks until they were splintered and stabbing at his tongue.
He licked his bottom lip until it bled.
He bit at his hangnails and cuticles and exposed sensitive layers of skin underneath; ones that burned when he spilled rubbing alcohol on them.
The only non-destructive way to keep his mouth occupied was to bury his face in Sam’s mound and lap at his clit until he squeezed his head between his thighs. It made Dean happy, and it stopped Sam’s brooding.
It was a perfect solution.