Joan is mad that Peggy has been able to work her way up the ranks of the company on the value of the work she does and the mind she has for the business, meanwhile whenever Joan brings something smart to the table it's either passed off to someone else as an idea or treated as this kind of wondrous moment.

Peggy is mad that Joan has achieved, and in some cases has always had, autonomy as a person solely because of her appearance. Peggy can't just show up somewhere and get a guy to do something because she looks good, she can't sleep her way to a million dollars. She has to grind past midnight in a lonely office cubicle just to live in a shabby apartment on the rough side of town.

Neither of them see the world from the other's point of view, and the competitive nature of that particular office helps enhance it. In Peggy's eyes Joan will always be the girl that had one hand in Roger Sterling's pocket while they held fairly similar positions, and Joan will always see Peggy as the girl who didn't have to sleep with anybody (and wasn't ever asked) to get what she wanted.

Though Don thinks she has great legs.

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"This is the streets, and I am the trap." � Jay Bilas
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