In Death of a Salesman, why does Biff come home in the Spring?

From the text:

And whenever spring comes to where I am, I suddenly get the feeling, my God, I'm not gettin' anywhere! What the hell am I doing, playing around with horses, twenty-eight dollars a week! I'm thirty-four years old, I oughta be makin' my future. That's when I come running home. And now, I get here, and I don't know what to do with myself.