A period of psychological depression (hereafter referred to simply as Depression with a capital "D"), is like being on heavy drugs. This should be obvious to those who understand the biochemistry of Depression. Consciousness altering drugs work by binding to the receptors of neurons or by increasing/inhibiting the production of chemicals that bind to the receptors of neurons. Depression is a disease where the "normal" mix of brain chemicals that bind to neurons (or assist in the propagation of synaptic signals) is abnormal.
To me, being depressed is much like an overdose of Benadryl the extends indefinitely. I feel like there is a crushing pressure on my skull, much like one would feel while diving deep below water. I feel like I weigh 10,000 pounds. In addition to this, my sensory perception changes. The colors almost go away. More exact would be that I feel like the meaning of the colors has gone away. Red transforms from a vibrant color to a simple designation that something has the property of being red. The effect that red has on the mind disappears, as does the effect of all the other colors. The same thing happens to taste. Everything tastes bland. Nothing feels pleasurable. Future dreams vanish and the past distorts as if a filter has been placed between oneself and anything good that ever happened. One's memory is altered to recall on the negative. The positive is inaccessible.
Feeling that way for a day might be like having a bum trip, as someone on acid might say, but imagine feeling that way for months or years and finding that nothing you do will change it? Would you want to go on living if life had absolutely no pleasure, and the world was dim, grey, tasteless, and irritating as you slog along carrying 10,000 pounds on your back and your head feels like it is imploding? I doubt you would wish to go on when you find that it is not something you can "snap out of." It is, in fact, a bum trip that will not end.
One of the best metaphors for depression is the movie "Melancholia" by Lars von Trier, inspired by a depressive episode he suffered. Here are some scenes from the movie that impressed me as true metaphors for depression.
And later we have a conversation between two sisters, the blond represents depression and the brunette represents the thinking of the "normal" person.
The sad thing is that the blond sister is correct and the "normal" sister is delusional. Sometimes I wonder if Depression is what occurs when biology takes away all of the psychotic drugs that the body produces in order to make the suffering of life go away. Without the naturally produced chemical stew that makes colors seem meaningful, chemicals seem tasty, and life seem hopeful, all that is left is the basic reality that existence is futile.
I share this with you because I think more people need to understand those differently minded from themselves. I'm not ashamed of having passed through many periods of Depression over my life, only of the stupid things I've done when I felt that way. It is only because I wish others not have to cope with my Depression that I have done something about my Depression. I've traded this raw dose of reality for what you call "normal." I did it for you, not for me.