Like, literally all day. (You can tell what kind of week it's been. XD)
But they just get more interesting the longer I look at them, because like, what word even describes Dean's mouth in these? Pucker? Purse? Image searching both will get you frowny!Obama, but what Dean's doing is a little more specific than that.
This is, of course, eternally an issue of the English language versus body language, particularly when you're a fan fic writer trying to describe Sam and Dean.
And then where does Dean's side-on come from here? What language is that? Because this isn't the only appearance of sideways!Dean--he makes that sideways face when the police are incredulous about his supposedly being Mr. Yamashiro's son in 1x05, and definitely other times. (Which now I want to track down, but I am NOT the person with that skill set. XD) Maybe Dean's vision is just better in that eye and it's reflexive; he might not even realize he's doing it.
Or: side-on standing is often considered invitational body language--versus standing and talking to someone at a head-on 90 degrees. When you're interacting with a dog you don't know, you're also supposed to stand side-on. In both instances, side-on standing is meant to belie dominance.
To take that even further, it can also be a tactically defensive approach, since you're reducing your surface area as a target that way--even if in this case the threat is still mostly hypothetical, and basically just some page on Sam's laptop; it's still the job, and habits hold. (Meanwhile, S11!Dean in the second GIF dgaf/isn't that concerned, and is also drinking a Beer of Relative Non-concern. XD)
Sidenote: When you approach something side-on (or something approaches you) your vision translates that relative motion as movement across the visual field, vs. front-on approaches, which translate it as expansion (the thing getting bigger/nearer). So your ability to judge the position of a thing should be better front-on--but according to Approaches to the Study of Motor Control and Learning (ed. J.J. Summers--did I mention I've been in meetings all day?) the difference isn't really statistically significant, and probably has more to do with practice (e.g. we usually perceive things head-on, and are therefore better at it) than position.
Anyway, back to the Beer of Relative Non-concern. I think it's adorable, for some reason, that Dean makes the effort to switch what hand he's using to hold his beer in order to lean over Sam's left shoulder. Why does he prefer the left? Since in ten years (the above GIF) that apparently hasn't changed.
Sure, maybe that wooden thing is in the way--I can't actually tell the depth of anything from the GIF, but it needn't be literally in the way to feel in the way, I guess. And that wouldn't hold for the top GIF anyway.
And SURE, SURE, you could say "because blocking." Similar to the wood thing ~feeling in the way, S1!Dean goes left because of the way the scene is lit; if he went right he'd put Sam in shadow. Ergo, "because blocking."
On the other hand, Dean Winchester doesn't know what blocking is:
And of course I can't think about benign over-the-shoulder-looking without thinking about less-benign looming:
Also on the left. Though I think in this scene Dean is basically just circling Sam the whole time, so maybe that's equal-opportunity looming.
ANYWAY, THE POINT IS. These GIFs are enchanting and the headcanon rabbit holes they can lead to are fascinating.
It's stuff like this that, for me, really answers the question, "So why do you write fan fic?"