It's rude to not do something you promised to do.
Jeff's friend Josh is having his twenty-first birthday party and Jeff volunteers to be the designated driver.
Jeff wants to have a good time.
Jeff spends the night in lively conversation with Josh, and drives Josh home after closing time.
Jeff decides to have a few drinks during the birthday celebrations and ends up getting too drunk to drive.
Jeff and Josh's friendship grows stronger in light of Jeff being a reliable friend.
Jeff and Josh are forced to walk several miles home from the bar at the end of the night.