POSITIVE and NEGATIVE are Metaphysical conceptions.
To be Positive is to be warm, friendly, kind, uplifting, supportive and Loving.
To be Negative is to be grim, depressingly realistic, unfeeling, indifferent, unkind, and unloving, and also, just plain cruel or hateful.
So, a Positive Story is preferred, when someone is making a list of something like
BOOKS---TO----RECOMMEND for readers.
Negative stories would be the books that are about the grim realities of life, or stories of depressing cruelty or human failure, or Man's---Inhumanity---To----Man and so forth.