I've had a headache off and on now for around 100 hours. The bursts of pain are conspiring together with a brain in a restless, roving mood to keep me awake.
Emily, in what is hopefully merely an example of the availability heuristic at work, has started to have the thought that I might possibly have a brain tumor. (Her job involves working with cancer patients). This long of a headache is unusual for me—the first occurrence that I can remember. The plan we came up with is for me to arrange to see a doctor if my head still hurts after a full week has elapsed.
We rely on the constancy of people's personalities for a lot of things. A friend or loved one's personality serves as a baseline for evaluating many things about them. If she is louder or quieter than normal, she might be feeling emotional. If he is facetious 90% of the time, he's probably facetious this time too. A person's personality might be part of what attracts us to them romantically.
There's a model of marriage that Emily told me about where you think about your marriage in terms of getting to know your "spouse of today." In this model, you assume that both you and your spouse's personality will change over time. Given that assumption, you then make it one of your goals to meet and get to know the slightly new and different person that your spouse has become, every day.
I imagine that even if someone were trying to get to know their spouse every day, a sudden personality change like from a brain tumor being removed, would be really emotionally difficult.