I dated for 5 years a woman in the same situation as what you describe. She has money, friends, family, a roof over her head. She hasn't suffered childhood trauma or anything abnormal, but she's consistently depressed and lethargic.
I'm bipolar myself. I've been through some difficult things, but my condition makes me acutely aware of how subjective human emotions are. How much of our thinking is influenced my neurochemistry rather than rational thoughts or memories. I'm pretty stable these days, but I've had times where I would just wake up in the morning and feel happy to be alive. Energized, stress-free, and confident in my abilities. The next day, I wake up depressed and I feel like my life is shit, not worth living. A few days later I feel fine again. No major life events, no major causes of stress, just my neurochemistry being unpredictable.
I think that the overinsistence in some circles on past trauma causing mental illness is harmful. There is definitely such a thing as genetic predispositions. Some depression and mental illness just does not have a cause you can point to. It's not all about overcoming negative experiences and willing yourself out of it. Genetic defects can cause all sorts of physical problems, just think of psoriasis, asthma, acne and peanut allergies. I'm not sure why people think that the brain can't have such issues. Believe me, people with peanut allergies won't overcome that through psychotherapy and confronting their mommy issues.
But all my research and experiences support the notion that both one's experiences and one's emotional reactions to those experiences are crucial. This is why you can have some people living seemingly perfect lives and winding up depressed, and others living through war and oppression and coming out fine.
In your friend's case, the environmental factors could be: (1) pressure from outsiders to "keep up appearances", (2) the negative messages and images that proliferate through the media of war, poverty, murder, terrorism, climate change etc, (3) a world that offers so many options of how to live but little consistent guidance about what is best, (4) living in a society that can still be very difficult for women, due to career challenges, threat of abuse, paternalistic attitudes etc.
These are just hypothetical of course, given that I don't know her.
And for her to be crippled by depression in response to these or other stimuli would be as much to do with a difference in the way she responds to such things vs the "average" person.
But it may not even be unhealthy. Maybe her true calling is to work as a humanitarian helping the needy rather than doing what she's been trying to do, and her depression is her mind's way of making her realise she needs to make that kind of change.
Having gone through this process in myself and with others, when you dig deep enough, something like this always turns out to be there.
But my main point is that the mind does not and cannot function independently of a person's environment, and paths to mental wellness that really work (and they do exist), involve addressing both one's inner reactions to things and one'e outer environment.
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.
- 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.
I think that someone can be affected by somethings in their earlier life that can affect them forever. I'm not talk about repressed memories, I'm talking about the lack of positively formative experiences at critical moments of their life.
Example - what happens when say someone starts school or is a teenager at the same time their mother is undergoing menopause and their father has lost their job and they're both starting to drink? The child will feel a lack of something but not sure what. If they do think of it all, they'll think of it as partly their fault.
How can you be sure it's not WiFi radio waves messing with her brain?
Can you really rule out space aliens trying to stop her from reaching her potential?
There's a reason we use the scientific method.
Could it possibly be that there is more to life than these things?
...could it be that there is more to life than these "objective factors"?
But she suffers off-and-on depression. Not just "I feel bad" depression. Crippling, "can't get out of bed" depression. Periodic suicidal ideation. Occasional suicide attempts.
She knows she has depression. She talks about it. And she wants to be done with it. She knows how she feels when her brain is "in balance" and how it feels when she's depressed. One of these states is much more preferable to the other.
She's tried drugs, ECT, talk therapy, etc. And, yes, she's tried ketamine. Anything that will put her back to a state where she is able to get out of bed and function, and reduce the odds of her killing herself when she's depressed.
I'm sure there are a lot of people who are depressed due to outside factors -- poverty, oppression, war, etc. But I can tell you quite certainly that there are people who have "perfect" lives who find themselves clinically depressed. And at least some of those people want to get back to a non-depressed state. This research helps them.
TL;DR: Not all people are depressed because their life sucks, nor is depression something that all people can "cure" by thinking harder or trying harder. Some people have a brain imbalance and would like to fix it so they can have a life. Literally.