Dexter finds the spring "dismal" because it means he must return to his work as a caddy, and abandon his "winter dreams." The fall, however, is "gorgeous" because then he can reflect on his "fleeting brilliant impressions" of summer and imagine himself, for example, a champion golfer, easily able to defeat...
the wealthy men for whom he caddies.
These "winter dreams" all have to do with the acquisition of things. The narrator explains that:
He wanted not association with glittering things and glittering people—he wanted the glittering things themselves. Often he reached out for the best without knowing why he wanted it—and sometimes he ran up against the mysterious denials and prohibitions in which life indulges.
Another way to understand the "dismal" nature of the spring is to associate it with the "mysterious" denials of real life. Dexter is a person who seems able to make his dreams come true; he becomes wealthy very young, and soon is living out his fantasy of playing at the club where he once caddied. Yet Judy Jones is one thing he cannot have. She is his most persistent "winter dream," a vision of loveliness that, like a dream, consistently eludes him. The reality of Judy is a bit more complicated, however. Whether Judy really is "the best" or not is beside the point—for Dexter, she is one of those "glittering things" he wants, and his inability to get her points out his own emptiness. The fact that Dexter desires her but cannot have her suggests that the "hope" he finds in the fall could be as empty as the "dismal" spring.