I agree with Michelle that we shouldn't ignore systemic issues in the workplace such as environmental cues (lighting, desk set-up) and interpersonal interactions (management influence). While I agree that people have choices over their mood I also find this to be a problematic topic. If you take a person who has historically been subjected to systemic social problems (poverty, racism, lack of education) and is disempowered in the workplace and say to them, "Don't worry, you can control your own mood, just think happy thoughts," then the core issue is not being dealt with. The systems that maintain this level of inequality need to be changed. Work "happiness" is a complicated issue and needs to be addressed from a systems perspective. Just my two cents.