To find the source of optimism you need to look at the end of Act I. This "Florida idea" as it is called begins when Happy mentions that Biff is going to ask Bill Oliver for a business loan. The plan is that Happy and Biff are going to use their sporting and physical abilities to market and sell sporting goods. Everybody thinks that this is a great idea - the two brothers going into business together.
Of course, relating this to one of the key themes of the novel, we can see that this is another example of the myth of the American dream. Their optimism is tragically moving by the end of the play.
At the beginning of Act Two, there is optimism in the atmosphere because Biff is meeting with his old boss, Oliver, to get a business loan to open up a sporting goods store with Happy in Florida. Happy and Biff plan to call their business The Loman Brothers and refer to their plan as the "Florida idea." While he is eating breakfast, Willy is excited at the thought of his sons going in business together and running a successful company. Willy and Linda are also optimistic about their future because Willy plans on meeting with his boss, Howard, to discuss his job. Willy wants to ask Howard if it will be possible to let him work in New York rather than travel on the road every day. Willy is sick of constantly traveling and has not made any sales lately. He hopes that his boss will grant him an opportunity to work in New York which is another reason why he is optimistic at the beginning of Act Two.