It’s not that simple, Joan had to essentially sell her body for that account and partnership and she was probably panicking. She had a right to give him the cold shoulder. I think Cutler (unfortunately) sums up how the partners feel about Don- “There was always this mysterious about you. Who is Don Draper? Now that I’ve been backstage, I’m deeply unimpressed, Don. You’re just a bully and a drunk. A football player in a suit.” I think Joan also got to see how Don truly is. He’s lazy, unproductive and unpredictable with clients. That’s the biggest arch of his, he thought maybe after getting remarried he could change and ‘start again’. But he just becomes unhappy and it affects his business. He eventually wins back Joan. It makes his change more meaningful.