Whether provoked by misfortune or endogenously (from the inside), whether during childhood or later in life - it is all one and the same. A depression is a depression is a depression no matter what its precipitating causes are or in which stage in life it appears.
The only valid distinction seems to be phenomenological: some depressives slow down (psychomotor retardation), their appetite, sex life (libido) and sleep (known together as the vegetative) functions are notably perturbed. Behaviour patterns change or disappear altogether. These patients feel dead: they are anhedonic (find pleasure or excitement in nothing) and dysphoric (sad).
The other type of depressive is psychomotorically active (at times, hyperactive). These are the patients that I described above: they report overwhelming guilt feelings, anxiety, even to the point of having delusions (delusional thinking, not grounded in reality but in a thwarted logic of an outlandish world).
The most severe cases (severity is also manifest physiologically, in the worsening of the above-mentioned symptoms) exhibit paranoia (delusions of systematic conspiracies to persecute them), and seriously entertain ideas of self-destruction and the destruction of others (nihilistic delusions).
They hallucinate. Their hallucinations reveal their hidden contents: self-deprecation, the need to be (self) punished, humiliation, "bad" or "cruel" or "permissive" thoughts about authority figures. Depressives are almost never psychotic (psychotic depression does not belong to this family, in my view). Depression does not necessarily entail a marked change in mood. "Masked depression" is, therefore, difficult to diagnose if we stick to the strict definition of depression as a "mood" disorder.
Depression can happen at any age, to anyone, with or without a preceding stressful event. It can set on gradually or erupt dramatically. The earlier it occurs - the more likely it is to recur. This apparently arbitrary and shifting nature of depression only enhances the guilt feelings of the patient. He refuses to accept that the source of his problems is beyond his control (at least as much as his aggression) and could be biological, for instance. The depressive patient always blames himself, or events in his immediate past, or his environment.
This is a vicious and self-fulfilling prophetic cycle. The depressive feels worthless, doubts his future and his abilities, feels guilty. This constant brooding alienates his dearest and nearest. His interpersonal relationships become distorted and disrupted and this, in turn, exacerbates his depression.
The patient finally finds it most convenient and rewarding to avoid human contact altogether. He resigns from his job, shies away from social occasions, sexually abstains, shuts off his few remaining friends and family members. Hostility, avoidance, histrionics all emerge and the existence of personality disorders only make matters worse.
Freud said that the depressive person had lost a love object (was deprived of a properly functioning parent). The psychic trauma suffered early on can be alleviated only by inflicting self-punishment (thus implicitly "punishing" and devaluing the internalised version of the disappointing love object).
The development of the Ego is conditioned upon a successful resolution of the loss of the love objects (a phase all of us have to go through). When the love object fails - the child is furious, revengeful, and aggressive. Unable to direct these negative emotions at the frustrating parent - the child directs them at himself.
Narcissistic identification means that the child prefers to love himself (direct his libido at himself) than to love an unpredictable, abandoning parent (mother, in most cases). Thus, the child becomes his own parent - and directs his aggression at himself (=to the parent that he has become). Throughout this wrenching process, the Ego feels helpless and this is another major source of depression.
When depressed, the patient becomes an artist of sorts. He tars his life, people around him, his experiences, places, and memories with a thick brush of schmaltzy, sentimental, and nostalgic longing. The depressive imbues everything with sadness: a tune, a sight, a colour, another person, a situation, a memory.
In this sense, the depressive is cognitively distorted. He interprets his experiences, evaluates his self and assesses the future totally negatively. He behaves as though constantly disenchanted, disillusioned, and hurting (dysphoric affect) and this helps to sustain the distorted perceptions.
No success, accomplishment, or support can break this cycle because it is so self-contained and self-enhancing. Dysphoric affect supports distorted perceptions, which enhance dysphoria, which encourages self-defeating behaviours, which bring about failure, which justifies depression.
This is a cosy little circle, charmed and emotionally protective because it is unfailingly predictable. Depression is addictive because it is a strong love substitute. Much like drugs, it has its own rituals, language and worldview. It imposes rigid order and behaviour patterns on the depressive. This is learned helplessness - the depressive prefers to avoid situations even if they hold the promise of improvement.
The depressive patient has been conditioned by repeated aversive stimuli to freeze - he does not even have the energy needed to exit this cruel world by committing suicide. The depressive is devoid of the positive reinforcements, which are the building blocks of our self-esteem.
He is filled with negative thinking about his self, his (lack of) goals, his (lack of) achievements, his emptiness and loneliness and so on. And because his cognition and perceptions are deformed - no cognitive or rational input can alter the situation. Everything is immediately reinterpreted to fit the paradigm.