"Only A Biker Knows Why A Dog Sticks His Head Out Of A Car Window". what does this quote mean?

Because a biker is exposed to the wind the air & is out there in the scenery. He knows what it's like to ride along without being in a box that cuts you off from the world & makes you feel like you're watching it go by on a screen. Someone who has always driven a car wouldn't know that rush, but a dog understands it, or at least just likes the feel of the wind in his ears!