I have all the physical symptoms of depression, and I lack serotonin, I usually need some bitter chocolate to get me moving but I am NOT depressed.
How do I know? The first and principal symptom of depression is LACK OF INTEREST. Depressed people don't feel like doing anything, they just don't care. If you ask a depressed person what they want to do, they would say nothing. If you ask a CFS person what they want to do, they would probably list 100 things, including jogging, studying, travelling, working full time, skying, and many other things. The problem is that we physically can't do it. We want to, but we can't.
In my case, if i were depressed I wouldn't be seeking illnesses and researching treatments to get well. I WANT to get out of this, and then comes the frustration with the doctors and their squares minds, but frustration doesn't depresses me, it kinds of angers me. But even with my lack of serotonin, i still have the inner motivation to keep fighting in the hope of finally finding a treatment that helps me.