Sam likes the idea of them being matching, of being able to put his hand on his tattoo and feel his heart beating near, and know Dean’s heart is doing just the same, keeping him alive and healthy and going strong.
Dean has his other hand on his thigh by now, fingers white where he’s digging them in, and Sam is hit by the sudden absurd urge to take it, to twine their fingers together and let Dean take his pain out on Sam.
Adult, incest. Tattoo pr0n. Gah, this is hot like burning! All the way from Sam not telling Dean what exactly the tattoos are for to the immediate aftermath of their first time.