Having a body again is a concatenation of bizarre, unfamiliar experiences. Freaky things going on with Sam's body are the least of his worries, though. Once he's as sure as he can be that Lucifer's still in the pit and that there's no trace of Sam's escape or of Adam or of whoever got Sam out, once he's established that Dean's alive and safe and with Lisa, Sam books a motel room near Cicero for a week or until the pickpocketed credit card gets frozen while he sorts things out in his head. On the one hand, Dean mourning Sam; on the other, Dean on the way to happiness without Sam. It isn't till Sam goes looking for a bar to hustle in (whatever he does next, he'll need money to do it) that Sam realizes that he hasn't eaten or drunk anything in the week since he got out. The need for it has gotten lost in the marvel of feeling temperature and pressure again and in the need to know what the hell happened, and with neither his body nor other people cuing him, it simply hasn't occurred to him to eat.
A little experimentation proves that if Sam's going to die of anything, it'll be decapitation or old age. The bullet to the head is enough fun that Sam doesn't want to try decapitation, and, since Patrick's corpse turned up on the local news while Sam was waiting for a blood test right after that poker game, Sam doesn't know how he'd test old age. The fact that Sam has tried multiple methods of suicide in the past several hours tells him that he's already decided Dean's better off never knowing Sam got out, though, especially since Dean's as mortal as ever. Better for both of them to make a clean break.
Then, well, the cousins, the djinn, Dean, and, well. Sam's good enough that the question of what's keeping Sam alive hasn't come up once while the family's looking, and if Dean wants to hunt with Sam, Sam's not strong enough to deny him. But then. Then the vampire hunt, and the vampire getting the jump on Dean, and Sam—Sam realizes. If Sam's time on earth is no longer limited, there's no reason Dean's should be.