In this quote, F. Scott Fitzgerald is explaining one of the many wonderful things about literature. Literature is a powerful force in the world, but it is often hard to explain why so many people are captivated by it. Here, Fitzgerald puts it simply—literature brings people together through universal emotions.
When he says that “you discover that your longings are universal longings” he means that when you read you realize that other people have the same feelings that you do. For example, it is very common for people to long for adventure. When we read a book in which a character also longs for adventure we feel understood. We realize that our feelings are normal and we begin to connect with and learn from characters who are feeling the same feelings as us.
Once we realize that our emotions are universal, we realize that we are “not lonely and isolated from anyone." Sometimes we might be ashamed of our feelings or feel like no one else has ever felt the way we do before. But then we pick up a book and realize that those feelings are not only normal but a key part of being human. This realization fosters a sense of solidarity with the rest of society, a feeling of belonging.
This quote brings to mind something the writer James Baldwin once said, which captures the same idea:
You think your pain and your heartbreak are unprecedented in the history of the world, but then you read.