I would consider the person involved and the circumstances. Once when I helped someone by giving her a ride when her car broke down, she did nothing but brag about herself and stink up my car with her perfume the whole way.

Also, my car has a lot of dog hair floating around, so not everyone wants to be subject to it (allergies, personal preference, etc.).

Dogs and nature abhor a vacuum.
http://geaugadoggy.wordpress.com