There are many systems of interaction between brain, body and social world that can get caught in positive feedback loops. Depressed people, for example, can start feeling useless and burdensome, as well as grief-stricken and pained. This makes them withdraw from contact with friends and family. Then the withdrawal makes them more lonesome and isolated, and more likely to feel useless and burdensome. Then they withdraw more. In this manner, depression spirals and amplifies.
