Many dogs experience fear and anxiety when placed in a vehicle. It may be that the only time your dog enters a vehicle is for a trip to a stressful experience (the Vet, for example). It may be that your dog is bewildered by the whole event, and has never learned that a car ride is not a threat. It may be that they have suffered from nausea due to fear or motion sickness.
This manual will teach you how to desensitise your dog in order to achieve more enjoyable car rides together.