Dogs are considered man's best friend for a reason. Research has shown that dog owners have more active lifestyles, lower blood pressure, and are less likely to get sick than their dogless counterparts. One study has even shown that people who engaged in petting sessions received a boost in mood-enhancing hormones such as serotonin and oxytocin. Although these benefits come with the cost of food and veterinary bills, many are more than happy to oblige.