Why are some employees disgruntled no matter where they work?

Happiness comes from within, and many people don't realize they even HAVE a negative attitude! Also most managers are UNWILLING to address uncomfortable issues with their subordinates or coworkers. But it's important that managers address an employees' negative attitude, otherwise it can create a toxic or unpleasant work environment for everyone else.

Not enough pay.

Some employees appear to be working in a profession that they dislike or are not really suited to perform and feel stuck - thus inflicting their malaise on their coworkers.

That's a good question...... I honestly have no answer.... Maybe they are just unhappy people.

treatment from management and/or demands like mandatory overtime.

Greed and jealousy, it has to do with the individual’s value system. Lack of discipline at their learning stages in life. They developed an irrational sense of entitlement, and possess distorted social mirrors

People get caught up in what everyone else is doing versus focusing on themselves. Some feel entitled to certain privileges or believe they are better than everyone else, which fosters negative behavior.

Some people have unresolved personal issues that keep them unhappy.

Because they dont want to work, i guess. Some people are lazy, others feel they 'deserve' to have life given to them. Fact of life is most people have to work, so find something you enjoy and get it done.