DPDR, especially if it's severe, chronic, and long-lasting, can be a very lonely, terrifying, and depressing experience. I think it's pretty normal and rational to sink into despair and sadness and hopelessness from having to deal with this for so long, with little support and understanding from friends, family, and the medical community.
Others may experience the "depression" that comes with this differently. After all, DPDR is often experienced as "no mood" by many (although I think we all do experience mood, it's just either muted, distorted, or extremely internalized. A person may want to cry, but find themselves unable to, for example). There's also the experience of anhedonia which is frequently experienced by those with DP and often categorized as "depression", but is very different from what most people think of when they hear the word "depression".