It's bad not to pay people what you owe them.
Bart sees a concert coming to town that he wants to attend, but his rent is due, so he borrows the cash from his friend Gary.
Bart wants to go to the show.
Bart enjoys the band and pays back Gary next payday.
Bart goes to see the band, but fails to pay back Gary.
Gary appreciates Bart paying him back promptly and is willing to lend to him again should he need it.
Gary no longer trusts Bart and will not make the mistake of helping him again in the future.