Negative capability and writing

In my English classes I learned that a factor in what scholars consider good writing is that it can mix the negative with the positive and still be readable.  I think it shows greater depth and realism to do such.  It is like a character who is complex has often both sides.  I believe we all have some kind of dark side and life has a dark side, but we should always to point in the lighter brighter direction if we can.  Negativity is stressful.