On a personal level I agree, but the manager responsible for dealing with changing requirements will not get very far by telling people to manage their emotions

On a personal level I agree, but the manager responsible for dealing with changing requirements will not get very far by telling people to manage their emotions better. It will come off as condescending and likely make morale even worse. If you're looking to change the way people think, know that it is incredibly difficult (sometimes impossible) and can only be done bit by bit over time by helping the individual to see a different way of looking at things.