Take Brett for example, she almost epitomizes the Parisian culture of the time period, which seems to be what Jake idealizes, but his relationship with Brett is obviously unhealthy. This becomes even more clear when, while she is gone and he is in Spain, Jake seems content for the first and only time in the novel. The contrast between his emotional health in Paris and Spain is highlighted by how well he sleeps and how he feels before and during going to bed in the two countries. In Paris, when his unfortunate relationship with Brett is not something he can ignore, he often sleeps extremely poorly and spends long periods of time thinking about his situation with Brett, sometimes crying because of it; not what you would expect from a person in a healthy relationship. In Spain, when Brett is removed from his thoughts, he is placid and his only thoughts before going to sleep are that "It felt good to be warm and in bed", he is content.
Jake's love of bull fighting also represents both how Spanish culture is a better fit for Jake, and that Paris is unhealthy for Jake. Jake's love of bull fighting seems to stem for it's tradition and it's passion, which are also seen in other elements of Spanish culture (i.e. the peasants with their more traditional approach to drinking); when he is in these settings he is much happier. This would seem to indicate that he would fit well into Spanish society, and that he would be content with it. However, when Parisian culture comes into the equation in the form of Brett, he goes back to being unhappy and loses touch with his values. Instead of being placid and happy like he had been before, he became unhappy and corrupted Romero, who basically symbolized everything Jake loved about bull fighting. Basically, Parisian culture is harming Jake's life.