An imbalance in this bag of chemicals which is our body.
If someone has a broken leg, you would be sympathetic abd not say 'don't be silly, of course you can dance if you put your mind to it'.
Like being in a black hole/tunnel and not always even being able to see daylight, let alone any way out.
Wanting the world just to bugger off wholesale and leave you the hell alone
You know how they say if you have a cold, if there was a fifty pound note on the end of the bed, you could reach it, but if you have flue, you don't care enough to even try? Depression is like that versus the 'we all feel a bit down sometimes'.
It's not a case of mind over matter. That's being a bit 'blue' or sad. Depression needs no 'reason'. You can't 'snap out of it', or choose to feel happy. Who chooses to feel suicidal?
Its hard to understand unless you've had the Black Dog gnawing on your ankle.