applying reason to the situation only leads to madness, as reason cannot resolve the situation

Perhaps the protagonist is doing it wrong.

If you try to use reason and do it wrong, it leads to suboptimal results.

I didn't read the novel, so I may be missing some important data, but in a reference group "many partners, the relationship is always great at the beginning, and boring later" the typical problem is that the person does not understand the difference between dopamine-based love and oxytocine-based love, and falsely believes that only the first kind is "true love" and that it should stay forever.

Based on this knowledge, some solutions could be suggested, such as: a) try to find some value in the later stages of love, or b) decide that you only value the former stages, and accept that the price for this is changing your partner every three or six months.

Hollywood love is just as stupid as Hollywood rationality.