Negativity is more infectious than positivity

One happy person can sometimes make someone else on the project team a little more upbeat. But one unhappy person can bring down the mood of everyone else, by being cynical, disruptive, and selfish. The challenge is how to help an unhappy person understand that the person making them most unhappy is themself. Sometimes it's easier to choose to be unhappy - by acting powerless - instead of having the courage to find and exercise whatever power you do have.
After all, once you know that you're unhappy - and why - you can channel your frustration and anger into making change happen.