Joe Keller grew up in poverty and was not able to attain much education. The Great Depression added to his fears of not having his material needs met. For him, his good job is all-important. He has an intense dread of being poor again, and he wants, above all else, to provide a good life for his family, with a nice home and steak dinners. He expresses his love through what he can give materially to his wife and children. A bit of the bull in the china shop, he doesn't think too hard about how he is making his money or if he should cultivate other values.
Joe has done well in providing for his family as a successful businessman. His tragic flaw, however, is placing too much importance on his job and financial success. He approves the sale of the cracked cylinder heads to the army even though he knows they are flawed because he wants to protect his job and family. He places his own security ahead of human lives.
Joe's vision is tragically narrow. Only the needs of himself and his family are real to him. The fact that other young men just like his sons are died as a result of his decision is not something he is capable of fully realizing until the end of the play.
Joe buys into a tragic version of the American Dream that says that material success is all-important, even more so than human life.