David got schooled in taxes after he told Johnny about all his tax write-offs. After throwing around "write-off" over and over, Johnny explodes and tells him these things aren't write-offs. Whoops, David clearly doesn't know what a write-off is and assumes it just means he doesn't have to pay for it. His adorable cluelessness is endearing, especially because he's really trying to do something with his life, and we love that for him.