Some of the best television comedies in recent memory have dealt with people struggling to come to grips with and/or redefine their identities. Consider Arrested Development, and how almost every season revolves around the characters' efforts to redefine the people they are. Whether its Tobias' constant struggle to redefine himself as an actor or George Michael's leatherbound efforts at establishing a more badass persona, characters in Arrested Development were constantly working to adjust their own identities.
Just like real people, characters in a sitcom are creatures of routine. While regular ol' human beings like routine because it helps life to be just a bit more manageable, it's a requirement for characters in a sitcom because that's the nature of the particular medium's story structure. That neighbor has to burst into the apartment in a particular way. This catchphrase needs to be said at the appropriate moment. These characters need to wind up at that coffee shop because it's the set we built and we can't invent a new place for these people to hang out every single week because we don't have all of the money in the world. Even when it's not a logistical choice (and it often is), repetition and routine are important to the sitcom because episodic TV more or less demand familiar settings and situations as latching-on points.
As It's Always Sunny in Philadelphia gets on in years, some of its best episodes have come from the shaking up of these routines, and even better episodes tend to come when the gang confronts these routines. Particularly, the 12th season finale "Dennis' Double Life" involves most of the gang coming face to face with they way they've been doing things for so long, and many don't really like the answer when they actually think about whether or not it's been working for them.
Dennis opens the episode by bursting into the bar and letting the gang know that he is already well in the middle of his own episode, having made the decision to pretend to be someone else long before the episode even begins. Weirdly enough, putting on a false identity is exactly what the normal Dennis Reynolds would do...which is why it's so interesting when the episode ends with him deciding to reject his shitty ways and try to be an actual responsible, caring human for once. Dennis' moment authenticity (and we'll probably have to wait until next season to learn if it was actually earned) comes when he literally stops trying to be other people.
Mac, on the other hand, jumps at the chance to be someone else, outright saying, "I don't want to be me; can I be someone else?" He desperately wants a new identity (something he has been trying to do for almost the entire series, and on which he has actually made progress this season)...but when he's given an opportunity to make actual changes in his life, he just winds up going way the hell out of his way to make his old apartment look exactly the way it has for the majority of the series so far. Charlie even dips his toe into new territory. Like Mac's, Charlie's narrative gets pushed forward in a serious and irreversible way when he finally sleeps with the Waitress...but neither of them can truly shed their skins and have anything other than an antagonistic relationship with each other. In fact, Charlie quickly realizes that the only thing he hates more than being ignored by the Waitress is getting nonstop calls from her.
Thematically the episode pushes some solid ground — yet another positive effect Megan Ganz seems to have had on the writers room — but still feels a little disjointed and out of step with the rest of the season. The jokes don't hit quite as hard, and the whole thing felt a little disjointed and stilted. When they all sat down for Frank's indecent proposal (which was a little too gross for my taste, if I'm being honest), I had to rewind and skim through the rest of the episode to remember how we got there. Turns out it isn't really explained. "Dennis' Double Life" ends on a solid character note, and has great character beats throughout, in which the Always Sunny gang take real looks at the people they are, and we see how they react to real opportunities to change those things. Unfortunately, all that character work came at the expense of pacing and jokes, making this a relatively uneven capper to the most consistently solid season of Always Sunny that we've had in years.