We've all seen it: a dog with his head tilted up, the breeze blowing through his fur, with an unmistakable look of pure joy on his face as he takes a ride in his favorite person's car.

The sight is enough to bring a smile to anyone's face. Everyone can identify with that feeling of unadulterated bliss. But what is it about car rides that makes dogs so happy?

The truth is, we'll probably never know for sure, because they can't tell us! But we've dug into the mystery a little bit, and found five plausible reasons for this curious phenomenon.

They like adventures. 

This doesn't apply to every dog, of course (some dogs are shy and crave comfort) but in general dogs are highly curious by nature and really enjoy any kind of adventure, from taking a new hiking trail, to meeting a new canine friend, to exploring that interesting-smelling crawlspace under the house. A car adventure is as good as it gets; it's novel; unpredictable; longer than many walking adventures; and offers all kinds of opportunities for an exciting time. Where will we go today? Who will we see?

There's a smorgasbord of smells in the air. 

Dogs are all about smells, and for good reason. According to Nova's website, scientists have determined that dogs have about 3,000,000 olfactory receptors in their noses, compared to our paltry 30,000. Dogs’ sense of smell overpowers our own by orders of magnitude — it's 10,000 to 100,000 times as acute. What does this mean, exactly, in terms of the difference in our abilities? Well, as dog-cognition researcher Alexandra Horowitz explains in her book “Inside of a Dog,” while we might be able to sniff out a teaspoon of sugar in our coffee, our pooches can detect a teaspoon of sugar in a million gallons of water. So, when you crack open the car window, you're cracking open a whole fascinating, unseen world for your dog full of novelty and intrigue that could keep him entertained indefinitely. If he's happy sniffing out familiar scents in your backyard, imagine how delighted he is to sort out thousands of new smells every minute, from the scent of flowers in bloom, to the smell of food cooking at restaurants, to lingering traces of a deer that ran by two hours ago.

There's something new to see everywhere.

Dogs are easily stimulated by visual cues. You've probably noticed this if your dog's ever gone manic at the sight of a squirrel or rabbit in the yard, or if he sounds off each day as soon as he spots the mail truck. On a car trip, there's no end to the things that can catch your dog's attention, from squirrels, to pedestrians, to other doggie passengers. Unlike their human travel companions, who are busy trying to get where they're going, dogs have the luxury of gazing out the window throughout the whole trip.

They're comforted by the close presence of their "pack.”

A car creates close quarters, and this makes dogs feel secure. They can see you, smell you, and be near you, while still getting all kinds of sensory stimulation they wouldn't at home. In a sense, you are a pack on an adventure together — just the way things would be in the wild. Except you're traveling in a familiar vehicle, with comfortable spaces your dog can curl up in if he gets tired of looking at the scenery. 

It triggers their hunting instinct.

Kevin Behan of Natural Dog Training believes that dogs love car rides because it puts them in the same state of euphoria that hunting does. The idea is that the motion of the car causes synchronized movements among the "pack members," as they sway along with the twists and turns of the road. These synchronized movements trigger a feeling in dogs similar that of moving in unison with a pack on the hunt. In addition, to your dog the objects outside the car appear to be moving quickly, which can trigger his instinct to give chase. 

Of course, it's important for dogs to be safe during car travel. Their heads and paws should stay inside the car at all times, and they should be properly secured for their safety and the safety of other passengers. This doesn't mean they will enjoy the ride any less, they can still see the sights, sniff the wind and feel the excitement of a new adventure. It just means you'll enjoy the ride even more knowing that they're both happy and safe.