We all knew Don would cheat on Megan. Don cheats on everyone! He lies and charms and philanders and seduces, and at the end of it all he's the tragic figure, not the people he cruelly betrays and discards. That's how Mad Men works. In life we often romanticize other people's perceptions of us; I want to be the man she thinks I am, or I like how I look through his eyes. Don goes further than that, though — he doesn't just want to be how he seems to the women in his life. He wants to be them, to take on their personalities and vice versa. What happens to them happens to him, and he wants to experience their emotions as his own. But sometimes that backfires, and it sends him running right into the arms of the next seemingly better option.
"This woman really knew me," Megan bragged on last night's episode, after a fan of her soap opera approached her in Hawaii. Pack your bags, Mrs. Draper: Don does not let strangers know him. People get to know Don only when he wants them to, and even then only barely. (Just ask Ken Cosgrove, who doesn't even know if Don's mother is alive or dead. When asked, he was met with total rejection.) If some rando Minnesota lady knows Megan, it means she knows Don, too — and that means if you listen closely, you can hear the fat lady singing.
We've seen Don's romantic transitive property from the get-go. "I don't make plans, and I don't make breakfast," Midge told him in the pilot. Neither did Don; when we were introduced to him, he was flying by the seat of his pants, trying to conceal his identity without any real strategy. (Lucky for him and unlucky for the scheming Pete, Bert Cooper didn't care.) Betty's superhuman levels of denial were matched only by Don's — This is working, this can be fine, I am fooling everyone. He said that living with Betty was like living with a little girl, but he was just as petulant and capricious as she was, just as punishing, and, in plenty of ways, just as helpless. No one was winning this maturity contest.
Don wanted to be Rachel Menken, too: someone taking over a family business, someone whose self-possession was rooted in familial and cultural identity, and someone whose religion came with the concept of a "homeland." (Recall that Don was working on a pitch for the Israeli tourism bureau.)
Maybe it was time to be more calculating, more sophisticated, to be bitter but certain. Just like Bobbie Barrett. "Does it make you feel better to think that I'm like you?" Don spit at her in a fight. But he was very much like her, just as ambitious and callous, certainly, as fascinated by power plays and as willing to lie. Later, he's fascinated by the carefree, seemingly untethered Joy in California. Who wouldn't want to be that comfortable topless?
Then there's Suzanne, Sally's teacher; she's forthcoming and energetic and believes that the things she teaches her students can have a positive impact on the world. It's enchanting to Don — I could be a source of good, he thinks. I can be someone who helped my brother instead of sent him off to kill himself. Being with, i.e. being, Suzanne is Don trying to right some wrongs, though again, it doesn't go as planned and he winds up leaving her in the car outside while he and Betty begin to end their marriage.
In season four, Don dates the bright young Bethany, even as he chugs along in his miserable bachelor apartment. He sleeps with Allison — Maybe I can be someone helpful. (Nope.) He sleeps with prostitutes — Maybe I can be someone invisible. (Nope.) Finally he meets Dr. Faye, and he thinks, Maybe I can be stable. It lasts for a little while, since Dr. Faye's specialty, and Don's specialty, is figuring out what people think, what they want even when they won't say it. Maybe I can be a person who really investigates his interior life. But Faye thinks she's being charming when she tells him that if he confronts his demons and decides to "resolve" some of his issues, he can then be "stuck trying to be a person like the rest of us." That's the last thing Don can do. He can't be like everyone else, and instead of that line being a moment of sweet understanding, it echoed in Don's ears like a threat. Which is what lead him to Megan. Perky, enthusiastic, casually but not oppressively maternal, roll-with-the-punches, have-a-good-time Megan. He tells Faye that he has no idea how to be a good parent, and then he watches Megan effortlessly forgive a spill at the table. I'll be like that next.
And now the shine's off her, too. At the end of last season, as Don watched Megan's audition reel, it dawned on him that she craved being beheld. She wants to be gazed at, admired, and it forced Don to wonder if he wanted those things, too. The answer was no — just look at how uncomfortable he is having his photograph taken in his office. He was Megan for a while — loving, present, more tolerant of other people's shortcomings — but now that doesn't work anymore, and he wants to be Silvia Rosen. Wistful, intellectual, married to a Jew (like he would have been had he settled down with Rachel or Faye). Silvia seems resigned to her spouse's demanding job, while Don wishes he could be; instead, he resents every second Megan is at her soap-opera gig.
So is Silvia going to be Don's true love? It seems unlikely. Because Don has yet to embrace the advice everyone seems to tell anxious middle-schoolers and reluctant OkCupid users alike when it comes to love and romance: Just be yourself.