This question has been circling around discussion for quite some time now. All of the lonely nights when Don should’ve come home and eaten dinner with his family, he instead was having multiple affairs. He was never working late at the office, like he so often told his wife. The funny thing is, Betty knew what Don was truly doing the whole time. As viewers, we would see the looks that Betty gave Don when he would come in late hours of the night, but Don didn’t see them. Betty is keeping all of her frustration towards Don’s unfaithfulness bottled up inside and it is driving her mad.
Why does she do this? She does it for two reasons. One reason is because of her children. She and Don have two children together who are too young to grasp the concept of why their parents don’t live together anymore. She needs to stay with Don for their children’s sake, and she doesn’t want to mess up their “perfect little lives” which is living the American dream - husband, wife, children, dog and a beautiful house. She also stays with Don because she doesn’t want to be judged by her friends.
Betty is looked at as the ideal wife in her circle of friends. She cooks, cleans, and takes care of the kids while her husband is off working and making all of the money. To them, Betty has no real issues because Betty never discusses her real problems with them. Her friends come to her with their problems because they know that she has all of the answers and can help, but Betty doesn’t practice what she preaches. She goes to see her psychiatrist, and at the end of season 1, she finally tells her shrink that she knows Don is cheating. For her, she must keep this image of the “perfect woman” but deep down she has many insecurities and issues. Don never puts his family first, but she continuously supports Don and “fakes the funk," meaning, she plays dumb but really knows the truth. How long will she keep this up? What will cause her to actually leave Don and move on? Something’s got to give and her leaving before Don got home in the last episode of Season 1 may be an indication of her making a step in the right direction.