Do you know people who just seem mad at everything on the job. Nothing is ever good. The get fixated on thing that can't be changed overnight. They are constantly bitching about thing they can't control, to the point that it affects their job. These people can tear a shift apart. 

Challenge these people get them to explain why they are so angry all the time. At the end of the day, it may be time for those people to move on from the shift or the job all together.

Don't let negativity seep into you work environment. We have the best job in the world. Approach the job with you best attitude.