There is increasing evidence that the emotional and psychological benefits of pet companionship have physiological counterparts as well -- that pets improve not only the intangible "quality of life" but also improve human health. Research has demonstrated that petting and caring for animals - particularly those with whom a bonded relationship has been established - can reduce blood pressure and heart rate and improve survival rates from heart disease. Elderly people who have pets visit physicians 16% less often than do those who do not, and that dog owners, in particular, make 21% fewer visits. Contemplation of a fish tank during dental treatment and oral surgery can reduce anxiety.
The key to animal-assisted therapy, then, is that animals in general, and companion animals more commonly, are regarded by many as "man's best friends." Dependent, dependable, domesticated animals may provide people with unconditional regard, present perpetually juvenile attributes (neoteny) which stimulate innate nurturing responses, and offer a sense of mastery and constant, non-judgmental acceptance and companionship. They may serve as catalysts to social interaction and as bridges to interpersonal communication and attachment.source:
http://www.animaltherapy.net/Premise%20&%20Promise.html