I hesitate to agree with the author of this module that negativity is stronger that being positive; but I do totally endorse it's incredible dampening impact on any place, process or relationship. When I have students that continually exhibit the "dark cloud sydrome" I usually call them in for a chat. I always let them vent first, because they usually do have some legitimate concerns; but once that is over I always ask them if they think their current attitude and actions are helping to change the situation. Unfailingly, they admit that that isn't the case. After such a talk, I always watch for any positive changes they make in the ensuing weeks and am sure to give them lots of kudos to keep it up. We don't necessarily change a person's personality this way, but we definitely can have a huge impact on their behavior in the learning environment.