If negativity were a mental illness, I would’ve been hospitalized years ago. I am not a therapist, but it seems pretty obvious to me that the great swath of personality types present in humans means that some will be more positive than others. From a counseling point of view, I suppose the concern would be some degree of balance – if you perceive everything as negative, surely that is classified as depression. But as long as something brings you joy, as long as you are able to maintain relationships, then who cares how pessimistic you are?