Why is Willy so touchy about being interrupted in Death of a Salesman?
Willy despises being interrupted, because he has been ignored and discounted for most of his life. His sons do not take him seriously, nor does his boss, Howard, and neither does Charlie, his only friend. Willy is obviously delusional, and feels extremely powerless. While he does exhibit clear signs of paranoia, it is evident that there is little in his life over which he has real control. He is desperate for attention and some sense of control, but perhaps even more so, he yearns to feel needed and worthwhile.
When he wishes to give Biff job advice, he feels he is being interrupted, and goes a bit nuts over it. Even though Linda knows that Willy's advice for Biff isn't overly helpful, considering the specific situation, she accepts his criticism for interrupting him and offering her own suggestions. Later in the play, she tells her boys that, "attention, attention must be paid to such a man." Willy has no place in life, and tries to take his own several times with the gas heater and the hose.
In a life spent during which no one wants to listen to him, Willy is extremely needful for someone to quietly accept his words as useful and worthwhile, hence, he is touchy about being interrupted.