John, just because you're sad about the finale doesn't mean everyone else has to be. True, Don's proposal was a wild plot twist that seemed to come out of nowhere, unless you count Faye's admonition, in Episode 2, that Don would be married by the end of the year. (Poor Faye!) But from the moment Don slipped that shiny engagement ring into his breast pocket, it seemed troublingly possible that he would pop the question to Megan before the season was out.
Why did Don propose to Megan? John, you lay out four very plausible theories, but I'd like to offer a fifth:
Don is turning into Betty. Which is to say, he's turning into a child. In the sales pitch to the American Cancer Society, Don talks dismissively about how sentimental children are—"Have you heard their music?"—and proposes to exploit that sentimentality to turn kids against cigarettes. But throughout the rest of the episode, Don regresses. He abandons all that he has learned from his wise, grown-up girlfriend about how hard it really is to be an adult and instead pursues the fantasy of being the perfect family man, married to the loving mom he never had. It's no coincidence that his proposal comes right on the heels of that scene at the diner, where Bobby, Sally, and Don look shell-shocked by the spill—post-traumatic milkshake disorder!—and soothing Megan plays the anti-Betty.
And consider how Don explains his hasty proposal to Megan: "Did you ever think of the number of things that had to happen for me to get to know you? But everything happened, and it got me here, and what does that mean?" What happened, in case you're keeping score: Don slept with and fired his secretary, his other secretary died at her desk, his daughter ran away from home, his firm lost its biggest client, he smashed his Clio in a fit of rage, his oldest friend died and left him a ring, and his wife fired his children's one consistent source of comfort. It's a string of calamities. How romantic! This It-was-meant-to-be reasoning is sentimental, superstitious, and childish in the extreme. No wonder we hear "I Got You Babe"—kid music—as the show fades out.
This theory is really just a variation of your idea that Don isn't ready to be Dick, John; he's afraid to grow up. But the episode drew an explicit parallel between Betty and Don: Don is jumping into a quickie union just as Betty is discovering that hers is less perfect than it seemed. I loved her coquetteish, "coincidental" run-in with Don in Ossining, N.Y.; she's fed up with Henry Francis, looking to press "undo." And Henry's advice about Carla—"There is no fresh start! Lives carry on"—sounds like something Don needs to hear as well. Instead, he's off to Tomorrowland, more entranced with his perfect future than his gnarled present.
It's an intriguing move, making Don as childish as Betty; it suggests that Betty is not a pathologically bitchy anomaly, a lone case of arrested development. Season 4 showed us that Betty is emblematic of Mad Men's larger worldview: Most adults, faced with adversity, act like foolish children. It's a sign of how far Don has fallen that he earns these congratulations from Roger: "See, Don, this is the way to behave."
Michael, I'm interested to know what you made of the finale. Although the episode's developments surprised me, I liked it a lot more than John. In fact, I thought it was excellent. If Don were to trace an unerring path to recovery, to become a thoughtful, upstanding guy with good judgment in one fell swoop, it would be bad for the show (what would we watch next season, couples therapy with Faye?) and, more significantly, feel too easy. Don has demons. It's OK with me if he takes a few more seasons to puzzle them out.
Oh and finally, John, Michael: YOU WERE SO RIGHT. Joan kept the baby! I'm shocked. (Not surprised, though, that she's withholding photos from Greg; the better to fool you with, my dear …)
I don't want to ride an elephant,
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