Material success is very important to Joe Keller, primarily because he believes it is the means of protecting himself and his family. Keller is a living illustration of the American Dream—the belief that anyone, no matter their background, can rise to material success in this country if they work hard enough. An uneducated man from a poor background, Keller has built a successful business that he plans to pass on to his surviving son, Chris, sparing him all the hardship he himself has endured. As he tries to explain to Chris, safeguarding this dream is all-important to him:
You don't know how to operate, your stuff is no good, they close you up, they tear up your contracts. What the hell's it to them? You lay forty years into a business and they knock you out in five minutes, what could I do, let them take forty years, let them take my life away?
Keller has a "circle the wagons" approach to life, in which you protect yourself and your family because everyone else is the enemy. If you don't eat others, they will eat you. As Chris describes it,
This is the land of the great big dogs, you don't love a man here, you eat him!
Joe is a sympathetic character in that we understand what a struggle it has been for him to achieve the material success that makes his family's life comfortable and gives him pride. As the play points out, too, his thinking is no different from most people's, and he does very sincerely love his family. He truly believes that the best thing he can do for his sons is safeguard the family money, even at the cost of other people's lives.
His core motivation is not really the money, but the security it represents for his wife and children. He shows he has a conscience when he can make the empathetic connection to the other pilots killed due to the faulty cylinder heads: when he sees them as "all my sons," he realizes that he has failed to protect them, just as he failed to protect Larry. At the end, he tragically kills himself.