?

The placebo effect. 

The nocebo effect. 

Both are real. 

Do the math. 

The existence of those terms implies a lot of "real" things, aren't. That works in reverse, as well.

Most of the time, when you hear someone call someone else negative, they don't like that you don't believe what they believe. 

They swallowed the pill/story. 

You gotta believe in that pill, or you won't be in the story anymore.

Apply that to all facets of what we call "life". 

Belief is an interesting word.