Apathy
Both Sophie and Otto appear to be suffering from the same character flaw. While they are unhappy with their lives, they both remain passive about making changes. Neither wants to accept responsibility for attaining their own happiness, in their relationship or otherwise. For Sophie this apathy is literally manifested in her refusing to treat her feral cat bite. She sees her life crumbling around her, but she refuses to take that as warrant for change. Rather than actively fix her problems or improve her relationship, she prefers to silently indulge in self pity, just as she allows her physical wound to fester to the point of crisis. For Otto, apathy means detachment. Whether intimidated by the severity of his own thoughts or simply afraid of intimacy, he rarely shares a thought with anyone, let alone his wife. He has silently feigned disinterest for so long that he's started to believe it, but he's in the same boat as Sophie. In order to be happy and feel fulfilled, he needs to take responsibility for making changes and really engaging in life.
Conflict
While conflict appears in every story, it is a particularly relevant theme for this book because of where it appears and does not appear in the story. For instance, relationships, especially intimate ones, are generally characterized by conflict. You won't fight as much with anyone on Earth as with your spouse, so it's interesting to note Sophie and Otto's utter lack of conflict. Instead of attempting to work through the obvious plethora of criticisms they have for one another, they preserve an attitude of disinterest between each other. On the other hand, Otto engages in an ongoing conflict with Charlie at work. Seemingly he bottles up all of his criticisms of Sophie because he doesn't care to fix the relationship and then he turns around at work the next day and loudly voices as many criticisms as possible at Charlie. Although Otto and Charlie do have real reasons to disagree, much of their conflict seems to be displaced from Otto's disappointing marriage. This reversal of the normal rules of social engagement and division in life demonstrates one consequence of Otto and Sophie's general apathy.
Stubbornness
Another reason Sophie and Otto are having issues in their relationship is because of stubbornness. While a reasonable person will participate in an argument because they agree to be willing to be persuaded, a stubborn person will not even entertain the argument. Each certain that they are correct, Sophie and Otto have already done all the talking they're willing to do. That's why they're now drifting apart. While originally each was willing to get to know the other person, now they have determined not to be changed by that person. Rather than continue to hear a different opinion, each of them prefers silence because they don't see any reason to engage in an argument; their mind won't be changed. Stubbornness like this is fatal to a relationship, which must be based upon open communication.
