Depression is vague feeling. It attacks like radiation poisoning. You feel different, far away, “off”, but you can't tell why. People will ask you, many times, “what's wrong?” and you'll be unable to answer. On paper, compared to the great majority, your life will be ideal. And any distinct misery you will have experienced – abuse, break-ups, job dissatisfaction – will not be not sufficient an explanation for the cloistering, all-over pain of a long-term disorder. It's hard, really, to explain “what's wrong.” And that, for many, is the handle - the struggle to articulate how this feels.
With other illnesses, the pain is localised and apparent, occasionally visible. With depression, it's everywhere and nowhere. It's in the taste of your food, the muscles in your legs, the itching in your scalp. It's in your sleep, your intercourse, your speech. But you can't reach out and touch it. You can't pick at it like a scab or wrap it like a wound. You can't reduce its swelling or hack it off, cough it up or sweat it out. Eventually, you reach a point of anhedonia, where depressive thoughts permeate everything and all. Misery becomes an element, like nitrogen or helium, silently present wherever you go. But still there's no open sore. How can you be ill? After all, you look okay.