The interesting thing is that depression while caused many times by a dramatic event.. The event itself usually causes the body to produce a chemical imbalance that has a negative affect (ie, continued depression) on the brain. While there are idiots out there that claim otherwise (like global warming), it is very much true.
The body is a biological entity which means everything it does comes from chemical reactions. Like a food recipe, if you increase, decrease an ingredient, it will change it's attributes. Add or remove ingredients, it too will change it's attributes. No yeast, a bread will not rise. In fermentation, sugar is converted to alcohol, but if you add too much sugar, the yeast will die once too much alcohol is created. This is why beer / wine doesn't get that much alcohol while spirits require distillation to reach higher levels of alcohol.
All chemicals cause changes. Not just while they are present, but over the long haul. Causing an evolution of sort within the body.