Why depression develops: the causes behind a depressive episode

Clients ask me the same question in different words within the first session: why me, why now. It is a reasonable question and it rarely has a single answer. Depression is better understood as a threshold that gets crossed when several ongoing pressures accumulate, rather than a single blow that knocks someone down.

A vulnerability that was there before the episode

Twin and family studies put the heritability of major depression at roughly thirty to forty percent, which means genetics load the gun without pulling the trigger. What seems to be inherited is not depression itself but a nervous system more reactive to stress, a temperament more prone to worry, and in some families a lower threshold for the mood networks in the brain to destabilise under pressure. A person with this loading can go for years without an episode if their circumstances stay manageable, and can develop one quickly once several stressors land together.

Life events, and the ones that matter most

Not all stress carries equal weight. The research on life events consistently points to loss, humiliation and entrapment as the categories most strongly linked to depression onset, more than events involving danger alone. A relationship ending with public shame attached, a redundancy that follows years of loyalty, a caregiving role with no visible exit: these carry a particular psychological signature, because they attack identity and remove a sense of control at the same time.

It is also common for the triggering event to look, from outside, too small to justify what follows. That mismatch usually means the event landed on ground already weakened by earlier losses the person had not fully processed.

Rumination: the mechanism that keeps mood low

Susan Nolen-Hoeksema's research on rumination found that dwelling repetitively on the causes and meaning of distress, rather than acting or problem solving, predicts both the onset and the length of depressive episodes. Rumination feels productive. It is not. It is the mind attempting to solve an emotional problem with the tools built for solving practical ones, cycling through the same material without ever reaching a decision or an action, and each cycle deepens the low mood it was meant to relieve.

I keep going over it hoping I will finally understand it well enough to feel better, and I never do.

Metacognitive therapy targets this mechanism directly, working not on the content of the rumination but on the belief that ruminating is necessary or helpful, which is usually what keeps the cycle running long after it has stopped producing anything useful.

The body's contribution: inflammation, sleep and the gut

There is growing evidence that chronic low grade inflammation, driven by poor sleep, sustained stress, a diet low in variety, and in some people underlying physical illness, can influence mood through pathways shared with the immune system. This does not mean depression is purely biological or that lifestyle change alone treats it, but it does explain why sleep deprivation, heavy alcohol use, an unmanaged thyroid condition or months of disrupted eating so often sit underneath a depressive episode rather than beside it. Addressing those physical layers is part of treatment, not a separate wellness add-on.

Attachment history and how relationships were learned early

People who grew up with caregivers who were inconsistent, critical or emotionally unavailable often build a working model in which love has to be earned through achievement or self erasure. That model is stable and invisible until adult life presents a loss or a failure it cannot absorb, at which point the collapse can look disproportionate to the triggering event because it is also reactivating something much older. Schema therapy and attachment focused work are often the right tools here, because cognitive techniques alone tend to move slowly against beliefs formed that early.

Why the combination matters more than any single item

• A genetic loading toward reactivity, on its own, rarely produces an episode without a triggering stress.
• A painful life event, on its own, is usually absorbed by people with a flexible thinking style and good support.
• Rumination turns ordinary sadness into a sustained episode by preventing recovery.
• Poor sleep and chronic physical strain lower the threshold at which the other factors become overwhelming.
• Early relational patterns decide how much the current loss is experienced as proof of something feared for a long time.

Understanding your own combination is clinically useful, not just interesting. It tells us where to put the effort: whether the work is mainly behavioural, mainly cognitive, mainly about sleep and physical stabilisation, or mainly about relational patterns that predate the current crisis. Two people with an identical PHQ-9 score can need very different treatment plans.

Where this leaves you

If you recognise several of these threads in your own story, that recognition is not an explanation you need to master alone. A proper assessment untangles which factors are driving your particular episode, and treatment can then be built around your actual causes rather than a generic template. I would be glad to work through that with you.

This article is general information written by a psychotherapist and is not a diagnosis or a substitute for individual assessment. If you are in immediate danger, contact your local emergency number.