It’s unusual to see characters wearing sunglasses in film or tv. Sunglasses exist in the real world to shield our eyes from bright light; but on screen, they only serve to conceal. They hide the most expressive part of a person’s face, so that neither we nor the person they’re talking to can see the truth behind what they’re saying. Sunglasses are therefore a mask for a character to hide behind, a fiction they’re presenting while hiding their true motivations.
Look at these sexy assholes with their sunglasses and their man-pain
From what I can recall, this is the first time we’ve ever seen Sam or Dean wearing sunglasses without them being a part of a costume (or a parody of a procedural). Presumably, they’re wearing them because they’re on vacation, relaxing in front of a lake, drinking beer, taking some “we” time in the sun. But because of the concealing nature of sunglasses, they aren't really wearing them to shield their eyes from the sun: their shielding themselves from each other.
Hmm, I wonder what that “No Hunting” sign next to them could possibly mean….
Both of them wear their sunglasses as they talk about how much they’re enjoying this little vacation of theirs, how they’re both feeling fan-freaking-tastic right now, how they’re both comfortable around each other now. When Dean casually mentions the possibility of a hunt, they’re still wearing their sunglasses when Sam says that they should just tell another hunter and continue to enjoy their vacation.
They talk about all of this while wearing impenetrable shields over their emotions, hiding behind reflective black lenses so that they don’t have to confront the implications of everything that’s happened, and what they really are to each other- brothers, or poison.
The only time that they remove the sunglasses is when they decide to go on the hunt, abandon the supposedly idyllic vacation that seems more tense and uncomfortable that relaxing. Dean removes his first to persuade Sam to go on the hunt; Sam removes his to make sure that this sudden glimpse into Dean’s real thoughts isn’t a resurgence of the darkness Dean has been suffering under.
“Hey let’s kill things, I swear I’m not still affected by the whole demon thing.”
“Sure, seems legit.”
(Seriously Sam, do you really expect Dean to tell if if things go wrong?)
As soon as Sam asks Dean if he’s ready for this, if going off killing things might be just a tad bit problematic given that Dean has just been off, well, killing things, the sunglasses come back on.
“I have literally never lied about being fine before.”
“Yeah neither have I.”
Of course Dean isn’t fine right now. You don’t just bounce back from becoming a demon and trying to kill your brother; that’s the kind of shit that sticks with you. And of course Sam needs to examine the things that he did trying to get Dean back, not just as things he had to do but as choices that he made. But the sunglasses are on and their masks are in place, and neither Sam nor Dean are able to reach the truth that lies behind them.
And even though they only wear the sunglasses for this one scene, the fact that they both put them on again before they leave their “vacation” is telling. It means that neither one of them is going to show their true selves in this episode, their real motivations and the real issues that are eating away at them and at their relationship. With those sunglasses on, they can’t see each other, and they can’t even really see themselves if they happen to look in the mirror. And until they take them off for real, everything that has happened between them is going to continue to rot and fester unobserved until it overwhelms them both.