In 1988, psychologists Shelly Taylor and Jonathon Brown published an article making the somewhat disturbing claim that positive self-deception is a normal and beneficial part of most people’s everyday outlook. They suggested that average people hold cognitive biases in three key areas: a) viewing themselves in unrealistically positive terms; b) believing they have more control over their environment than they actually do; and c) holding views about the future that are more positive than the evidence can justify. The typical person, it seems, depends on these happy delusions for the self-esteem needed to function through a normal day. It’s when the fantasies start to unravel that problems arise.

it's true. at least in china,if some patient who think himself is healthy,he will keep in good longer than someone else in the same conditions

sometimes things are just depressing,but you can choose your attitude towards them and a positive frame of mind is more beneficial