The unconditional love of a furry, four-legged animal can be just what the doctor ordered for depression sufferers. Not only do they provide companionship, but decades worth of studies have shown that petting a cat or dog can increase levels of serotonin and dopamine, chemicals in your brain that improve your mood. And if you feel calmer after petting your furry friend, it's because doing so also lowers blood pressure. Even if pet ownership isn't for you, you can benefit by visiting a local shelter or rescue where dogs, cats and even rabbits welcome volunteers.