Sam shouldn't have been surprised..but he was.
When rough, calloused fingers tenderly and ever so slowly ghosted over the flesh of his sweat-laden back he couldn't help but tremble.

When one hand slid up his taut throat then into his dusky-brown hair while another started caressing the growing bulge in his jeans a moan of 'yes' escaped from his parted lips.

When teeth sank gently yet possessively into his muscular shoulder he dropped the axe he was trying to grind, narrowly missing his foot.

Sam should have been surprised but wasn't.
Dean never could resist his brother when Sam was shirtless and trickling with sweat.