Here is a good article about the causes of depression from Harvard.

What causes depression? - Harvard Health Publications

It’s often said that depression results from a chemical imbalance, but that figure of speech doesn’t capture how complex the disease is. Research suggests that depression doesn’t spring from simply having too much or too little of certain brain chemicals. Rather, depression has many possible causes, including faulty mood regulation by the brain, genetic vulnerability, stressful life events, medications, and medical problems. It’s believed that several of these forces interact to bring on depression.

To be sure, chemicals are involved in this process, but it is not a simple matter of one chemical being too low and another too high. Rather, many chemicals are involved, working both inside and outside nerve cells. There are millions, even billions, of chemical reactions that make up the dynamic system that is responsible for your mood, perceptions, and how you experience life.

With this level of complexity, you can see how two people might have similar symptoms of depression, but the problem on the inside, and therefore what treatments will work best, may be entirely different.

Researchers have learned much about the biology of depression. They’ve identified genes that make individuals more vulnerable to low moods and influence how an individual responds to drug therapy. One day, these discoveries should lead to better, more individualized treatment (see “From the lab to your medicine cabinet”), but that is likely to be years away. And while researchers know more now than ever before about how the brain regulates mood, their understanding of the biology of depression is far from complete.

What follows is an overview of the current understanding of the major factors believed to play a role in depression.