You may know someone who is depressed and not know they're depressed. People expect someone who is depressed to cry a lot, stay in bed all day, mope, or sound like Eeyore from Winnie the Pooh. But depression isn't always this obvious.
Some people can totally fake it. They can smile and laugh; they can act like everyone else, even while they are in excruciating emotional pain. Occasionally people who can do this end up killing themselves, and no one can believe it.
The pain is also complicated by the fact that depression attacks a person's thoughts and feelings, rather than liver or lungs. Depression can cause a person to think she hates herself or is unhappy in her relationships.
It can cause someone to believe everyone would be better off without him, or even that others would be better off dead. It can cause people to feel sad, angry, guilty, numb, or rageful, even when none of this is how they feel when they aren't depressed.