There is plenty of research about how animals and pets affect our lives and our health. One CDC study says that having a pet can have positive effects on your blood pressure, your cholesterol and your triglyceride levels, can reduce feelings of loneliness, and can increase your reasons to exercise or socialize.
Here's an about.com article on how pets reduce stress:
http://stress.about.com/od/lowstresslifestyle/a/petsandstress.htm
And here's a great piece on WebMD on Five Ways Pets Can Improve Your Health:
http://www.webmd.com/hypertension-high-blood-pressure/features/health-be...
I have three collies and two cats (so I should be in perfect health, LOL!) and every time I see or hear about another "discovery" that pets enhance our lives, I almost want to laugh because it seems like such a no-brainer. Coming in the door and seeing a wildly wagging tail can wipe out the stress of a bad day. When I dig my hands into my collies' fur to pet them, I can almost feel my heart rate slow. And a happy, purring cat on your lap increases feelings of contentedness as well as making you think twice about disturbing her just to get up to get a snack!
Do you have a pet? What kind? How does she or he affect your day? What makes the expense and the hair on the furniture worth it for you? And would you be as healthy and happy without them?