Not only do pets have a therapeutic benefit for people with depression and ill health, but animals can also improve the circumstances that lead to depression and ill health.
Social isolation is a contributing factor towards depression, having a dog at your side may enable you to have contact with many more people and that in itself can be therapeutic and possibly decrease the risk of developing depression.

Pet therapy is also used to help people living in nursing homes and other long-term care. Having a close relationship with another person is thought to help depression. A relationship with a pet may have a similar effect.
Very few studies have been carried out on the effects of pet therapy on depression. Typically, these studies compare pet therapy with some other treatment or with no treatment. Owning a pet is a long-term commitment. While pets can give affection and companionship, they require the same level of care in return. This in itself is  self nurturing and emotionally rewarding.  Social isolation can be a contributing factor towards ill health and depression, then simply having a dog at your side is potentially going to enable you to have contact with many more people and that in itself can be therapeutic.

Some of the ways in which the 'golden' touch of pets is currently being used in Australia to benefit the elderly is through visiting or live-in pets in hospitals and nursing homes and also a program that the helps the elderly people look after their own pet in their own home.