It seems to be very common for people to hide it. Probably because mental illness has been such a taboo subject for so long. Also, these people are probably trying a "fake it till you make it" approach to life. They are hoping that if they at least appear cheerful on the outside, eventually they will genuinely feel that way. And there is also the fear that their friends will leave them if they reveal how bad they're feeling. Particularly if they're viewed as the fun one of the group.

Stephen Fry said that after his suicide attempt, his friends were baffled why he hdn't told them he was feeling that way. He said something like: imagine you had a horrible wart on your penis. Would you tell you friends about that?

Also, there is probably the doubt that they will be able to help. If the answer to "Hi, how are you?" is "the inside of my head is a shit filled black hole" or "my life is completely fucked and I can't do this anymore". Most people wouldn't know how to deal with that.

I suppose I'm just a bit skeptical. I've had friends who have had mental health problems. People who I've gone far out of my way to try to help. If only solving severe mental health problems was as simple as asking someone "How are you?" and listening to them. If only...