Why did Gerwig choose Merrily We Roll Along as the school play for the film? Well, the plot of that musical details three young, idealistic, very close friends who gradually drift apart, with the main character (Franklin Shepherd) particularly becoming obsessed with popularity and losing all of his old connections for newer, flashier people. That plot is a perfect mirror of Lady Bird's attempts to fit in with the "cool" crowd, though unlike Merrily, she realizes her mistake and comes back. To further drive the point home, Julie (the best friend that Lady Bird abandons) plays Mary (the best friend that Frank abandons).
Lady Bird and Marion's (her mother) relationship is strained throughout the movie, finally culminating in her silent treatment of Lady Bird when Marion finds out that she and her husband (and Lady Bird's father) kept the fact that they were working behind her back to apply for colleges on the East Coast. Now, this seems like an act of Disproportionate Retribution, and it probably is. But look at Marion's background for a moment. She has stated that her mother was an abusive alcoholic. Once you realize that, a lot of Marion's behavior makes more sense, especially since, from her point of view, Lady Bird doesn't seem to appreciate what Marion tries to do for her. In addition, Marion probably didn't have the things Lady Bird had growing up, and her husband losing his job is probably bringing back bad memories of her childhood. Furthermore, to add a bit of Fridge Horror to it, Marion acts (subconsciously or not) at times like her abusive mother probably acted towards her (kids of abused parents many times become like their Abusive Parents when they become parents themselves); watch the scene where Marion points out the cost of raising Lady Bird and then says that Lady Bird will never be able to pay it back because she won't be able to get that kind of job. Marion probably heard that herself from her own mother. This is the real reason for her breakdown after dropping off her daughter for her trip to college; she realized she was becoming like her mother, who she was probably trying to avoid emulating. Watch the film with that in mind ...
Likewise, Marion's letters to Lady Bird at the end, which provide an additional Freeze-Frame Bonus. A few things come to light:
Miguel's never-explained adoption: Marion stated that she was almost forty-two and so shocked when she got pregnant with Christine (additional Fridge Brilliance regarding the MacPhersons' pained financial state: Larry and Marion are in their late 50s and still have no retirement in sight; when most people should be relaxing, they're still living in a starter home). Considering Miguel is about five years older than Lady Bird, it seemed Miguel may have been adopted because Marion just couldn't get pregnant.
Marion tries to rationalize why she and Lady Bird do not get along, and in one letter, she muses that she may have been jealous because Lady Bird's face would always light up when her dad got home no matter how much time she spent with her mother. It also explains why she's so much closer to Larry as a teen.
In one of her shorter letters, Marion seems to resent that Lady Bird wants to call herself Lady Bird and not Christine, but she cuts it off quickly, indicating that she doesn't want to give her daughter more grief for her choice, but she clearly wishes Christine were prouder of her name.
Lady Bird accuses Danny of being gay after seeing him kiss a boy in a restaurant bathroom, and he confirms it ... despite the two of them having a relationship earlier in the film. What seems like a typical case of No Bisexuals makes more sense, though, when you remember that the teens attend a Catholic school—the only "education" they would have received about homosexuality was likely "it's a sin," and bisexuality probably wouldn't have even come up.
The movie is set in 2002-03. At that time, the family is struggling to make ends meet even with two working parents. At the end, they refinance their home to pay for their daughter's education, and Larry still doesn't have a job. They're also still in their starter home, presumably because the value hasn't gone up enough. One can only assume things won't look better in five years when the recession hits.
When Lady Bird goes to see Julie on prom night, Julie is crying, but won't say why. The obvious assumption is that she's upset she didn't have a date for prom. But we know about her crush on Mr. Bruno, and his solicitous friendliness tiptoes right up to the edge of flirting. And Lady Bird's been out of Julie's life for a while. One shudders to think that something untoward might have happened between Julie and Mr. Bruno, something that she's ashamed to tell even her best friend.
