My problem had been that I didn't want it to be something specific--cheating, infertility, workaholicness, etc.--mostly because Susan and Gary are going to stay together in the end and while I do think couples can overcome any of those problems, I also think that the act of overcoming would take over the story, which I didn't want.
So, here's what I'm thinking: Susan is basically questioning the value of being married, of dealing with the same person's B.S. for 50+ years. This comes up for a few reasons:
- At a fairly wild 30th birthday party for a friend, as a joke to harken back to their college days, they start playing the game "I never." (Just in case you don't know... someone makes a statement like, "I never had sex with a stranger" and people who have done that have to do a shot.) One of the statements is "I never fantasize about people other than my spouse". Gary drinks to that. And while intellectually Susan isn't shocked, emotionally she is. Drunk as she is, she asks him in the taxi home who he fantasizes about, and drunk as he is, he tells her its someone at work. She can't get that idea out of her head, and it makes her start to wonder if he wants to sleep with this person, then how is that so different from actually doing it? And what is marriage then?
- At the same time, she's been having some doubts about the marriage on a more emotional level: things have changed and he doesn't fulfill some of the needs he used to--like being thoughtful and making her feel special (by doing little things like bringing her candy bars that he knows she likes) and having long talks about random things like current events or the nature of people. They're busy, but she also feels like he's reluctant to try to reincorporate these things into their lives. She has found friends to fill this gap, but that makes her wonder why she is married if her husband can't be her best friend.
- There has to be a catalyst that makes her bolt. Maybe her mom dying is enough (life is short, etc.)