IWhy does Peggy despise Don again?
On the surface, she blames him for being the catalyst that drove away Ted Chough, the partner that slept with her, claimed to have been in love with her, then recanted at the last minute and said he couldn't leave his wife and kids for her. The Ted character took a massive shift from his early development. Early on, the character was kind of a slimy ruthless "do anything at all costs" type of manager. I think when Weiner got the big extension from AMC and the big dollars, he had to pad out the future seasons and then realized he needed Chough as a partner for a merger, and not an antagonist, so Weiner softened up the character, a lot of softening.
The other "open" reason is that she left the firm, mostly because Don threw the money in her face. Clearly she is "Don's girl" and her career, advancement and protection from the other partners comes from Draper. It was the final nail in the coffin that Don Draper would never accept her nor acknowledge her professionally, and that put the silent nail in her last hopes that he would validate her life personally.
Consider Peggy's situation, she's from an average no name family with no connections, and her Dad clearly ran out on her family, and all she could really hope for was a secretary job so she could meet a husband, but then the economics of attraction come in, and Peggy just isn't pretty enough to land a "good husband" on her own. Then there's Don Draper, her boss, her protector, the guy who gives her the big shot at a career and all the other men want to be Don and all the women want to be with Don ( Sort of a Austin Powers in a way)
If Don dated and married Peggy, it would mean that society would have to relent and acknowledge Peggy's worth and value openly. When a guy who has it all and could have anyone, but he wants you, it says something about you. From a very core level, this is also why advertising works. I.E. the association with the use of a product and the type of lifestyle it denotes and implies. They push this hard with Jaguar, pointing out it's a really lousy car that needs lots of maintenance, but that having one says something about where you are in life and how people should see you.
She loves Don because he acknowledges her and validates her as a person ( He gave her a career and has, for the most part, protected her as his protegee )
She hates Don because he will never acknowledge her and validate her as a woman ( He won't sleep with her, though he has plowed through plenty of women in the office, even marrying one, and even at his lowest and drunkest and most pathetic, he won't touch her)
The missing piece is the #### Whitman issue. It's the missing piece of information that would let Peggy have real peace with her situation with Don. Without it, it's incomprehensible to her why Draper would pick her out of the crowd and see that she has a real talent for understanding what people want.
If you look at character construction, most of the core Mad Men characters have socially desirable points, but not all. Joan is pretty, but lost her youth and has to fend for herself as a single mom. Pete came from a good family name but his legacy there was actually very hollow, while his name opened doors, he was always resented because it's all he had. Sterling had the luck of his father's advertising legacy, but without the real advertising talent.
With **** Whitman, who grew up in a whorehouse, raised by people who resented and hated him, and coming from purely nothing, Don Draper understand what it means to "covet", what it means to want something so bad that you would do anything to have it. Deep down this is why Draper is excellent at advertising. He understands what people want, but moreso, he understands the deep dark part within that covets but will never say so out loud. And by triggering that emotion, to go to that depth, he fundamentally drives what companies want from their consumers. To be caught knowing they have more than they need, but less of what they want, and finally being told and shown what they should want.
Don Draper is an "advertisement" for **** Whitman. It's the core reason why Don is so miserable. He sees how deeply and badly people covet what he has, but nothing he has is real. Thus Peggy is a kindred spirit for **** Whitman, not Don Draper. A girl who came from nothing and knows what it's like to covet, to want, to be accepted, and willing to do anything to get that validation and acceptance. Thus her clutching Don's hand in the first season, indicating her willingness to sleep with him, and her talk with Freddy, of "never wanting to be just one in a basket of kisses"
There are moments when Don Draper, a man with the pedigree, looks, wealth and talent, to have it all, treats Peggy in a way that only an ugly, poor, broken down, uneducated hick born in a whorehouse would. There's a certain kindness, a certain deference, a certain gentlemanly way that Peggy's other unsuccessful and socially undesirable boyfriends have, because they have no choice, but there is Don, at times, at moments, who gives it to her, in her mind, freely.
The point of "morality" on the show appears when one becomes into a state in which all advertisements fail. Pete gets his affairs and his NY apartment and he's still miserable. Joan gets a little partnership and what she thought was the right marriage and is still miserable. Sterling has everything but is empty inside and is miserable. Even with gains in their lives, the core characters are unsatisfied, ultimately because no thing, no achievement, no status, no promotion, no career, no financial security can cover up the empty pit of lack of self acceptance.
Peggy hates Don as a proxy for hating herself. Even if she could "get" Don, she would still learn to "covet" one more thing, and if she got that, it would be another thing. And again, this is why advertisement works, preying on the public's desire to covet and then to keep coveting, all a proxy to avoid oneself.
That's the bitter irony, what makes Don and Peggy the creative backbone for advertising at their firm roots from what makes them so miserable in their own lives. The problems exist not because Don and Peggy are so different, but because **** and Peggy are so very much the same.