Writing a well tones paper on depression requires a combination of variegated efforts directed toward a single purpose. On top of that, if you are writing a PhD dissertation on things that are not going too well, there are some things that will try to get to clicked right out of the way. Here you will have to fight the challenge of writing a high level academic paper in general and that of dealing with a lot of misinformation around the domain in particular.
There are some basic causes for depression that have been identified by the experts. There are also some other causes that are known to cause depression. You must go through all these causes in detail before you get going with the introduction of your paper.
The causes of depression also include some that cannot be generalized. They may act as personal causes for distinct individuals. You may skip these ones.
The one thing you can do to gain more knowledge about the condition in general is to meet people who are depressed and are not willing to recover from it. Some people just get addicted to the condition in general and may even refuse medical treatment. The obsession of a depression patient with their own medical condition is something that has not been talked about much.
There is a basic commonness in all people that are depressed. Most of the depression patients suffer from a very self-centered point of view and this is one reason they are often not able to come out of depression in a long time. They should rather make their voices heard and engage in social service. Comment on this aspect of depression patients in the dissertation.
If you have been through the condition before, you will be able to relate better with victims of the conditions. You will know about the struggles and advances of people who are willing to make the most of the available changes and understand what has been said about it in the past. There are also those who have not been through the condition but have seen such patients closely. You may speak to these people as well.
What makes things appear the same for the non-depressed? This is another question that you may consider brooding over.