How about I refuse to drive my roommate to work?
Why won't you drive your roommate to work?
Because he said he'd drive me to work.
You should give your roommate a ride if needed. The two of you could perhaps work out a schedule to alternate driving. This way you'll both benefit from the favor.
Nah, I'll tell him to ask next time.
I can understand that he offered first, but maybe something happened. If someone needs a ride, you should give them a lift if you are able. Maybe you can find out what happened.