This missive is not meant to be an exhaustive inquest about the physiological reaction to stimuli (i.e. emotions).
Emotions are so exigent, by their very nature, urging physical movement to bring about a different and/or complementary emotion; e.g. in my personal case, depression warrants joy or emotionless-ness. Depression is a tricky emotion to combat however because it is not always fleeting, and by its nature, is depressive whereby behavior is characterized by the lack of behavior. The urge of depression does not sway one toward resolution via equilibrium of mental state; instead, it encourages further degeneration until an ultimatum is encountered: die or change.
The motivation to change is fueled by the prospect of hope and/or fear, that is, an exclusion of one or the other or a combination of them both. I personally identify with the pair of emotions; for me, the will to live is based on the fear of others’ sadness caused by death and the hope that things could and might get better.
Summer days : winter days :: joy : depression