- running multiple ultramarathons with too little rest can cause depression, even if you love it. - working very hard in a job you like with too little sleep for a long time can cause depression.
In both cases it's likely your brain it just telling you that you're pushing too hard.
Some people -- like me -- find it easier to get into these mental overdrive situations, and some people might be more likely to suffer depression from the same stressors, but ultimately it seems to be the same factors at work.
I don't want to be preachy...one thing I've come to realize from many online discussions with other people suffering from depression is that almost every case is unique in some way and that the mental health community's efforts to try to group together what we're feeling into a single problem with prescribed courses of treatment is probably counterproductive. People believe that because they've experienced depression, either first- or second-hand, that they understand what all depression is like...astazangasta's comment is an example of this.
I try not to be one of those people, so what I'm offering is a suggestion, not an answer. My depression is by no means fixed, but meditation has helped me. It's one datapoint in case you want to try it.
It's not that I don't believe your depression is endogenous, I'm just curious. I was in a similar position to you until I looked closely at my life and identified the factors that were causing my depression/CFS.
Yet I still have episodes where I sometimes can't get out of bed, or can't bring myself to leave the house. A couple of months ago I spent three days just sitting (and occasionally sleeping) in a chair, not even getting up to eat or shower. I just simply couldn't do it.
While it's true that people can experience temporary depressive episodes due to 'exogenous' factors (e.g. the death of a loved one), it is very different to 'endogenous' depression, and requires different types of treatment. While talk therapy and time can be very helpful for people to work through difficult and temporary 'adverse circumstances', it is of little help for endogenous/biological depression.
No amount of 'talk therapy' can convince my pre-frontal cortex to operate within 'normal' parameters or to get my brain to produce appropriate quantities of neurotransmitters like serotonin, dopamine and noradrenaline. I'm reasonably sure this is a chronic illness that I will never be rid of (as I've learned from 15 years of failed treatment attempts). On the upside, there's a good chance that there is some treatment out there, coupled with sustained lifestyle changes, that will allow me to stave off the symptoms so that I can live a relatively 'normal' life. In some ways it's similar to how type 1 diabetics can survive and live normal lives with regular injections of insulin.