Why does he steal the money?

Paul's very much one of life's natural misfits. No matter where he goes or whatever he does, he simply doesn't fit in anywhere. As the real world is such a crushing disappointment, he constructs an alternative fantasy world for himself to inhabit, a parallel universe in which he imagines himself to be something better than he actually is.
Wishing to rise above his relatively humble origins, Paul seeks to emulate Pittsburgh's captains of industry, with their enormous wealth, culture, and sophistication. He feels himself entitled to better things in life, and recoils at the prospect that he will one day become a fine, upstanding member of the city's sturdy bourgeoisie, just like his father. But for a young man without wealth and social connections, leading the kind of life that Paul so desperately craves is nigh impossible. And as he's unprepared to do anything constructive that might actually change his life, Paul tries the more direct route of stealing to fund an opulent lifestyle. So he takes over $1,000 from his employers and heads off to New York City, where he—briefly—lives out his fantasy as a rich young man about town.