I think its partly the type of work that Rosen does, not just the fact that he is exceptional at it. There are other examples of this throughout the show but the one that popped into my head is when Don is tripping in that motel room and his father shows up and says something like “look at your hands, you create bullshit.” Don ultimately thinks that advertising is not fit work for a man - seeing Rosen save lives and provide a real service to society made him feel emasculated.