When the chairman of Madison Square Garden approached Sterling Cooper to spearhead the ad campaign that would result in the destruction of the original Penn Station, Don Draper observed that "change is neither good nor bad, it simply is. It can be greeted with terror or joy, a tantrum that says 'I want it the way it was,' or a dance that says, 'Look, something new!'" 
The Monolith, the room-sized computer that those of us in 2014 snicker at while tweeting from smart phones that have more computing capacity than was used to put a man on the moon in 1969, is met with consternation from the creatives whose bullpen is torn down to make room for this latest technology, but as with most things produced on Mad Men, it's a handy metaphor for what is going on inside our fictional advertising agency. Where once there was simple novelty at the appearance of a Xerox copy machine,  now copywriters sulk that their hangout is being destroyed and fear that their jobs might be next.
But Don, as he usually does, has it right - change is agnostic, it is how we react to it that matters. Adults once took for granted that their word would be heeded, unquestioned, and final. But the "grown ups" are discovering that their children were not as clueless as they assumed. Sally's eyes were opened to the seedier side of adulthood long ago,  but now as a teen, she is unafraid to challenge her father and call him out on his lies. 
So too is Roger and Mona's daughter Margaret, who has gone full native and shacked up with some hippies in upstate New York, shirking her responsibilities as mother and wife to lead a pastoral existence where there are no rules. When her parents try to bring her back, she cuts down her mother by reminding her that she spent most of Margaret's childhood nursing a bottle of gin. Roger, more simpatico with the live-and-let-live vibe, turns on his daughter when she sneaks off in the middle of the night to shack up with one of men at the commune,  only to be dismissed when he tries to forcibly remove Margaret. She calls him out as an absentee father who was never there for her when he tries to underscore her responsibility to her son Ellery. Roger may have a rotating cast of long hairs coming in and out of his suite at The Algonquin, and he's more than happy to dose and stand naked in front of the window,  but his conservative view of parenting runs smack dab into the counter culture and the result is a muddy suit.
Don's problem is quite the opposite. He is the one being treated as a child. He can't even reel in a small-time account without Bert tsk-tsking him and leaning into the fact Don was placed in a dead man's office. His former protégé is now his boss, directing him to turn in tag lines for a new client like he is some sort of junior copywriter and being summoned to her office for meetings. His reaction is predictable - he has a temper tantrum, reflexively reaching for a bottle of alcohol to drown his sorrows and stumbling out of the office in the middle of the day like the swinging dick he once was. It is not until the following morning when Freddy gives him the kick in the ass he needs. "What do you want?" Freddy asks. "I want my job back." Don retorts. And while Freddy's response, "Do the work, Don," may not be what he wants to hear, it is what Don needs to hear, because Freddy basically tells him the world has changed and if you want to get your job back, stop feeling sorry for yourself and go back and get it.
On the other hand, change has given Peggy more power but she is not mature enough to wield it properly. Having Don foisted on her Burger Chef account would lead a more experienced person to tap into Don's deep reservoir of talent and skill and treat him decently and humanely; but instead, Peggy's instinct is to humiliate and belittle him. Perhaps it is just desserts for the years of suffering she experienced, but having been handed an opportunity to take on a new, and high profile client, she is ill-prepared for how to respond - indeed, she views it as a suicide mission handed to her by higher ups waiting for either she or Don to crash, instead of a chance to shine. Lost in all of this conduct is the nuanced and complex relationship she and Don share. Perhaps she has taken to heart Don's long-ago admonition to forget about her childbirth and move on,  erased the night of Anna's death,  or just simply can't let go of the fact that she thinks Don destroyed Ted,  but instead of rising to the occasion, she has a tantrum of her own.
Of course, age is not determinative of a person's capacity for reacting well to change. Lou is no more mature than Peggy when it comes to Don's return. He feels threatened and is looking for any opportunity to kick Don while he's down. The rest of the agency seems to simply want to move on, with Joan dismissing Don's presence like that of an annoying gal in the steno pool, Bert telling Don the firm worked perfectly well in his absence, and the copywriters Don used to make shudder now conscripting him to move furniture with them. So it is not surprising that Don spent weeks sheltered in his office reading Phillip Roth and hanging that Mets pennant on the wall as a small reminder of Lane. No one likes to be treated like a child, but when Freddy tells Don to snap out of it and stop sulking, he is telling Don to forget the fact that his co-workers are treating him like shit, that Bert Cooper is pretending he didn't build this new agency, that Lou Avery would sooner pour gasoline than water on him if he was on fire, that his former secretary is now his boss, or that Joan, who he once counseled not to sleep with Herb from Jaguar,  treats him like a stranger. Do your fucking job. Be an adult. Embrace change. Do not be a monolith. We shall see how long this lasts.
PS - Pete adapts to change with lightening speed. He went from feeling remorse that his ex-father-in-law had a heart attack to glee over being able to "kill" him by signing Burger Chef, in a nanosecond.
1. Love Among The Ruins, Season 3, Episode 2.
2. For Those Who Think Young, Season 2, Episode 1.
3. See, e.g., Mystery Date, Season 5, Episode 4, At The Codfish Ball, Season 5, Episode 7.
4. A Day's Work, Season 7, Episode 2.
5. The rank hypocrisy regarding infidelity on the show is not limited to Roger. Don would not accept that Megan so much as kissed a man on her soap opera (To Have and To Hold, Season 6, Episode 4) and he shut down Betty's nascent attempt at re-entering the world of modeling (Shoot, Season 1, Episode 9).
6. The Phantom, Season 5, Episode 13.
7. The New Girl, Season 2, Episode 5.
8. The Suitcase, Season 4, Episode 7.
9. The Quality of Mercy, Season 6, Episode 12.
10. The Other Woman, Season 5, Episode 11.