The difficult part is that resentment doesn’t always arrive with an argument.
Your coworker might never directly say, “I think you should help pay for gas.” Instead, they may gradually become frustrated that they’re always driving while you appear to assume the arrangement will continue indefinitely. At the same time, you may begin wondering whether you’re taking advantage of their generosity.
A short and straightforward conversation can prevent both people from having to guess.
You could simply say:
“I really appreciate you driving me to work every day. I know gas and everything else adds up, so I’d like to contribute. What would be fair?”
That approach accomplishes something important even if your coworker refuses the money. It demonstrates that you recognize the ride as a favor rather than an entitlement.