External Conflict

At this point, I am rather confused about Jake and Brett's relationship. They speak of how they love one another, yet they say they cannot be in love. Why, you might ask? Good question, I have no idea. By the sound of it, Brett does not want to be in love with Jake but simply cannot help it.
"It's funny, it's very funny. And it's a lot of fun, too, to be in love." [Jake]
"Do you think so?" her eyes looked flat again. [Brett]
"I don't mean fun that way. In a way it's an enjoyable feeling."
"No," she said. "I think it's hell on earth." (page 35)
Brett does not find love as a source of happiness. Rather, she finds it as a source of suffering. When Jake returned to his room, he thought of Brett and began to cry. This further perplexed me. Why can't these characters be happily in love? This anguish demonstrates an external conflict. It is a conflict between two people in love. Perhaps the author will unveil the reasoning behind this is a near chapter.