After a lifetime of living in one another's pockets and knowing all of each other's secrets, it's just understood that there are some things that are off limits for teasing.
Sam's relationship with Jess. The whole Joint From a Guy Named Don incident. Anything related to any Winchester going to or being in Hell.
The one that would really surprise people is Dean's television watching habits.
He absolutely hates talk shows. The longest he has ever managed to watch one before calling the guest a whiny, self-righteous, entitled little pussy who needs to stop bitching about their problems was 94 seconds. The guest was Tom Cruise.
Don't even get him started on the Baby Daddy episodes.
He hates cop dramas, because he hates being reminded that people are much more evil sometimes than the things he hunts.
He hates reality shows, because he doesn't believe 90% of whatever happens on them is real.
He hates any show with mythology (especially Greco-Roman) because the writers always get the details wrong.
He doesn't hate horror movies, but he picks them apart, pointing out every inconsistency and inaccuracy until Sam hates to watch those with him.
He likes sci-fi, like Star Trek and Star Wars and Battlestar Galactica.
He likes westerns and action shows. But in all the years the two of you grew up in cheap motels with television for a companion, you've seen pretty much every John Wayne and Clint Eastwood movie half a dozen times, and have memorized the dialogue from some episodes of Bonanza and The Dukes of Hazzard.
He's not a fan of most sit-coms. He doesn't really like Seinfeld or Friends or The Beverly Hillbillies. The reason is not what most people would think. It's because many of their laughs come at the expense of making fun of someone else, and no matter how much Dean does it himself, it makes him uncomfortable to watch it on tv.
His favorites, not that he'll ever admit this under any amount of torture, are the shows and movies directed toward the tween and teen audiences. He'll watch Back to the Future and Bill and Ted's Excellent Adventure every time they come on. He watches the Disney Channel and Nickolodeon and The Cosby Show on TVLand. He likes seeing people smile and kids eat dinner with their parents and happy endings. He teases Sam about being Freddie from iCarly and asks where the girls who looked like Zoey Brooks were when he was in middle school.
If Sam wanted, he could humiliate Dean over his Hannah Montana habit.
But he doesn't. He just grabs a couple beers and flops down beside Dean to watch Drake and Josh.
There are worse ways to spend time.