Arthur Miller makes it clear that Biff really loves his father and his father loves him. But they just can't seem to get along. Towards the end of the play when  Biff tells Willy off, the love-hate relationship between father and son is brought out sharply. Biff tells Willy:

I am not a leader of men, Willy, and neither are you. You were never anything but a hard-working drummer who landed in the ash can like all the rest of them.

Finally:

Biff's fury has spent itself, and he breaks down, sobbing, holding on to Willy, who dumbly fumbles for Biff's face.

WILLY, after a long pause, astonished, elevated: Isn't that--isn't that remarkable? Biff--he likes me!

LINDA: He loves you, Willy.

Biff comes home to see his parents. He loves Willy and Willy looks forward to seeing him again. But then the conflict between them resumes. As Linda tells Biff:

When you write you're coming, he's all smiles, and talks about the future, and--he's just wonderful. And then the closer you seem to come, the more shaky he gets, and then, by the time you get here, he's arguing, and he seems angry at you.

Willy criticizes Biff for leading a vagabond life and not striving to achieve the big success that Willy has always expected and demanded. Willy asks Biff all sorts of questions and gives him all sorts of advice. Biff is a grown man. He doesn't want any more fatherly advice, especially since he can see how little Willy has achieved in life and how little his advice is worth. This naturally leads to quarrels and bad feelings. Biff goes away, but he comes back home again from time to time, hoping that things will have changed. (Many young people have had similar experiences when they go back home to see their parents.) Willy, on his part, also hopes that Biff has changed and is now ready to "take hold" and make a lot of money in the business world. Willy never really changes. He is too old. He is worn out and used up. But Biff can change because he is young enough to do so. He makes the irrevocable decision to start living his own life and to stop trying to please his father by being something he realizes he is not.

The answer to this question can be found in the conversation that Biff and Happy together have in their bedroom after they overhear their parents talking. Biff talks to Happy about his life and the way that it has run on a certain repetitive cycle over the last few years. He goes and gets a job on a farm, normally looking after horses, and then when spring comes, he feels an urge to return to the city and somehow try and make something of his life. Note what he says to his brother:

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.

Biff then returns every spring because he always begins to think of his life and what he is doing with it at this point in the year. He begins to feel dissatisfied with what he has done and achieved and returns to the city to his parents' home to somehow make it big. As the play continues, we see that Biff is just as trapped in Willy's dream of success as Willy is himself, but at least Biff manages to extricate himself by gaining a certain amount of self-knowledge about who he is.

At the start of the play, Biff has just returned home. He and his brother Happy mull over their lack of success and fulfilment in life; Happy chafes at being kept under in his low-paid office job in the city, while Biff has no settled work. He describes to Happy his latest spell working on a ranch out West:

This farm that I work on, it's spring there now, see? And they've got about fifteen new colts. There's nothing more inspiring or - beautiful than the sight of a mare and a new colt. And it's cool there nowm see? Texas is cool now, and it's spring. And whenever spring comes to where I am, I suddenly get the feeling my God, I'm not getting 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 making my future. That's when I come running home. (Act 1)

From this quote, we can see that Biff feels particularly restless in springtime, which heralds the blossoming of nature for another year (symbolised here by the mares and new colts), a time of new beginnings, a chance to start afresh. However Biff realises that his own path has stagnated, he doesn't have a proper regular job, he's making hardly any money, he's simply 'not getting anywhere'. This panicked realisation has brought him home, but once he arrives, he still remains without any direction, without any idea of how to build his life, his future. As he says himself, he is acting irresponsibly, he has no career and he's not married; he's 'just like a boy' (Act 1). 

Biff has become confused as to what he really wants from life and how to go about it. This stems from his failure in high school, his unexpected discovery of his father's infidelity (which he never reveals to anyone else) and his father's unrealistic notions about how to get on in the world, which he is forever trying to impress on his sons. Biff has come to resent Willy and to blame him in large measure for his own failures.

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.