Good news, dog lovers! Petting a dog for just 15 minutes can lower blood pressure by 10%. Spending time with your four-legged friend releases serotonin, oxytocin, prolactin and even lowers the stress hormone cortisol, according to findings compiled by Tombola. These feel-good hormones, like serotonin, lower stress and anxiety. Serotonin is the hormone antidepressants try to replicate.