Death of a Salesman Lesson Plan

How does the tone shift at the start of the second act? Why does it shift?

Willy now appears cheerful and optimistic. The boys are a blessing rather than a burden. Symbolically, the pipe connected to the gas heater with which Willy tried to commit suicide is now gone, and Linda presumes that Willy took it away himself. Things are looking up

But the sense of optimism that dominates the start of the act is somewhat unfounded. Everyone's hope is based almost entirely on Biff's meeting with Bill Oliver, trumped up in Willy's mind to a sure-bet business plan. Willy has gone from suicidal to confident and cheerful in the matter of one night, despite the fact that...