Dexter from this Fitzgerald short story is much like the author's most famous character, Jay Gatsby. They are both from Minnesotan families and long for a better, glamorous life away from their upbringing. Dexter begins working at a young age, showing his ambition. And, he knows that caddying for rich golfers is no way for him to escape his tedious life; so he refuses to caddy for a little girl.
Another example of his ambition is his willingness to pass up a sure thing (a business course that his dad could have paid for) in order to pursue a more prestigious education out East. Just like James Gatz, Dexter makes a plan for his future early on and successfully carries out the business part of it (if not the personal part).