Why does Biff come come in the spring?
Biff also comes home because, as he says himself, "I'm mixed up, mixed up really bad."
He loves it out west. It's beautiful there: idyllic, filled with the simple poetry of the outdoors and working with one's hands. But part of him thinks he's not getting anywhere, not building a future, not making any money.
He comes home and finds that his brother, Happy, is successful but depressingly unfulfilled and that his father has been exhausted by a life of driving and selling, selling and driving. He has the transcendent yearnings of his father as well as his need to be a success both at the same time.
Biff comes home to discover what is really important to him. He come home so that he can leave home of his own accord. He finally comes to terms with what he really wants in his own life and not what his father wants him to be.
Spring is the season of the melting of ice, rebirth and new growth. Arthur Miller brings Biff back home during the spring in his play, "Death of a Salesman," to symbolically illustrate this point. Other than his glory days as a high school football star, Biff has failed in nearly everything he has attempted. He is looking for a new start in life, so he has returned home to his roots in the hope of regaining some of the old Biff magic there. Although Willy wants him to remain at home, Biff knows a new start out west, where he can work with his hands (in the soil, where growth begins) will be the best way for him to get on with his life.