For decades scientists have been searching, without success, for a Grand Unified Theory of physics to fully understand phenomena like the Big Bang or the God particle. Meanwhile, a few years ago, Stanford researchers found a Grand Unified Theory of Dogs and People. (Well, actually, that's my name for it. They're still calling it a hypothesis.) They drew it out like this:

Now, it doesn't take a scientist to figure out that dogs, on the whole, bring people a sense of well-being. Whether you have your own dog, you greet your neighbor's golden retriever on the sidewalk, or you go to puppy therapy, you probably enjoy taking a few moments to snuggle up into Fido's furry coat and watch his glee as you scratch his ears.

But it turns out that dogs don't just make us happier; they also probably make us healthier. There is evidence—studies still conflict, but the data is becoming clarified—of a constellation of health benefits specific to dog owners. They are less likely to get nervous when put in challenging situations, and tend to have have lower blood pressure and cholesterol levels, as well as fewer heart attacks. And even when they do have heart attacks, dog owners are less likely to die in the year afterwards than the dogless. But why? For years, researchers struggled to explain the statistics. But then the Stanford group came up with its hypothesis, and it goes like this:

When people spend time with a dog, there is a tendency toward a very particular empirically demonstrated improvement in mood. This is illustrated in the graphic by the guy at the top, smiling and kneeling with Rover. In this mood change, anxiety and depressive symptoms decrease. Let's consider them both.

Anxiety has been shown to correlate with endothelial dysfunction. That's the failure of the inner lining of your blood vessels to constrict or expand at the right times to control the substances that pass through them. Endothelial dysfunction is connected to all kinds of cardiovascular diseases, including diabetes, high cholesterol, and blood pressure. (By the way, if you want to GIVE yourself endothelial dysfunction, a great way to do it is to smoke tobacco. So put down that cigarette and go cuddle with a canine instead.)

Depression, meanwhile, is associated with activation of your sympathetic nervous system—that's what's sometimes called the fight-or-flight response. Your pupils dilate, your heart rate increases, you sweat, your blood vessels constrict, and so forth. Fight-or-flight is how you feel when, say, you're in a calculus exam that you didn't study for. It's generally unpleasant.

So as depressive symptoms decrease, the sympathetic nervous system deactivates. That's how the scientists account for the fact that dog owners are less vexed by tough situations. When that deactivation occurs, your heart rate decreases, and you end up with slower blood flow to your kidneys, as well as to the ol' ticker. This has two consequences: Your blood pressure goes down, and your heart is less taxed, making a myocardial infarction (that's the fancy word for heart attack) more unlikely. In the graphic above, the brain represents the changes to your nervous system and endothelial dysfunction, and the arrows point to the various organs that undergo positive change when you chill with your pup.

Furthermore, dog-walking is exercise. Scientists have shown that during physical activity, our kidneys reduce the release of norepinephrine, which is a neurotransmitter and hormone that stimulates fight-or-flight. That means that your blood pressure goes down as vessels dilate and endothelial dysfunction gets reduced. That's why the lady in black silhouette walking a Chihuhaua has an arrow pointing toward a kidney. As fight-or-flight decreases, you become more relaxed—which is why so many people consider it a pleasure just to take their pooch for a spin around the park.

Now, all the mechanisms I've described can take place concurrently, and the consequence of it is that when you're with a dog, there's a positive feedback loop of sympathetic deactivation and lowered blood pressure, along with less depression and anxiety—and thus increased happiness. Furthermore, if you and Lassie are hanging out with other hound-lovers at the same time—for example, at puppy therapy, or with other dog-walkers you meet at the park while your furry friends sniff each other—you're giving yourself the added psychological benefit of positive social interaction.
So even though physicists may not have figured out their Grand Unified Theory, biologists have got a hypothesis about why puppies make us so happy. And that's a phenomenon that is, perhaps, ultimately more sublime and mysterious than the Big Bark (er… Bang) or the Dog (er… God) particle.