It is good to thank someone who drives you home.
Tom is very drunk, and Olivia is the designated driver for their group.
Tom wants to go home.
Tom says thanks when they get to his house and offers to return the favor.
When Olivia drops Tom off, he just leaves the car without saying anything.
Olivia thinks Tom is a good guy and helps him with other things in the future.
Olivia feels slighted and doesn't offer to help Tom anymore.