Why is it that depression is so random and hard to detect, that even the happiest person turns out to be the worst mentally?

Depression is to put it simply a 'mood disorder', for unknown reasons we can't shake off that constant overwhelming and consistent feeling of sadness and disinterest. It can get worse but there are also simple technqiues to ease it with meditation, therapy/councelling, self help...etc. Everyone is hiding a dark interior of sadness, as people who openly talk about this feel enviess of those that we 'think' are happier. That is why we must always keep an open mind, not everyone is feeling as successful or happy as we make them out to be in our heads and it is okay to go through this because - You are NOT alone :)