Negative emotions produce negative physical conditions ... which produces negative emotions ... which produces negative physical conditions ... and so it goes on in a descending spiral. Sometimes just time can heal the hurt and pain - however some people never really get over it.
Emotions use lots of energy and can cause a physical drain. Equally, physical conditions produce emotions. If you're tired, for example, you get grouchy. A physical ailment such as toothache can cause great emotional stress.