Suicide

Life is not conducive to making it possible for everyone to be within those "healthy parameters". So you're saying that if someone is unhappy because they are homeless, because they have a severe illness which reduces their quality of life to nothing, or if they are in debt over their heads to the notoriously vicious local loan sharks, then all of those people are mentally ill because they have a negative reaction to conditions that virtually anyone would find unpleasant. If you have to be alive, then of course being healthy and happy is the optimal state of mind in which to be. But being 'alive' isn't necessarily preferable to being dead, given that if you were dead, you would not feel deprived of any of the things that you enjoyed or may have enjoyed when you were alive.

Death is the cessation of any desire of need to chase the elusive goal of happiness and fulfillment, and it is completely rational to foreclose on the chances of having an enjoyable life if the conditions in which you are living your life make those outcomes unlikely. Considering the fact that one cannot be deprived of anything if one is dead, and that the pleasures in life mainly consist of relief from deprivation will lead one to the conclusion that dying peacefully is better than a life of suffering.

The idea that the worst life is always better than the best death is incomprehensibly absurd, and is the hallmark of superstitious thinking.