There are several reasons why Nick is not fully content with his life.
First, he’s “within and without” at the same time. He feels as if he’s part of the world around him and that he knows it well—but at the same time, he feels like an observer. This is because he’s a writer, and even though he describes himself as “enchanted” by his surroundings, he also feels like an outsider looking in on them. This makes him feel like something is missing in his life, which contributes to his dissatisfaction with it.
Second, Nick’s discontent is reflected in the way he describes how boats against the current can never reach their destination—they are always “borne back ceaselessly into the past.” This idea that our lives will always be defined by what has already happened reflects Nick’s own dissatisfaction with his life—he feels as though he has no control over where things are going or what they’re going to be like once they get there.
Thirdly, Nick describes Gatsby’s green light as “the orgastic future that year by year recedes before us,” which means that Gatsby sees something promising ahead but knows it will get further away from him every day until it disappears completely.
