"The Girl Next Door" revives several issues that have been lingering since the Pilot itself. While the boys recover from the Leviathan attack, Sam investigates murders linked to a case he solved as a teenager. The monster this time is a kitsune, and they feed on human pituitary glands to survive. But as Sam says, "nothing in our lives is that simple," and that holds true for this very case.
From the start, Sam has been trying to be normal, to get that "apple pie life" without hunting. It really isn't until this episode that we see how comfortable he has become with his life as it is, not what it cannot be or will never be. This contrasts the trajectory Dean is now embarking and has been on since season 6 started more or less. Dean's difficulty with "the life" could be traced back as far as season 2's episode "Croatoan" in fact.
Sam's buying of cake instead of pie for Dean is also symbolic here of the unattainable for both brothers. The "Apple Pie Life" is just out of reach, impossible to keep, and doomed from the start every time a Winchester attempts it. Sam has accepted this more or less by this time, Dean has not. It is tugging upon a thread that has long woven through Supernatural's fabric, and one that will probably never be fully resolved for either brother.