Bjorn Borg, Roger Federer , Pete Sampras and Ivan Lendl are players who never showed an inch of their emotions on court. (Sampras exception was in regards to his coach's death). Whether you celebrate a point with a showcase demonstration, or you show your disappointment at a point you lost, you are showing some weakness, some lack of control.

The above players killed opponents with their impenetrable mask. It made them seem unbeatable, that you couldn't worry them no matter what. The end of match celebration showed that these players were not ice cold, but that it was a decision they made to keep their cool no matter what. Furthermore, you spend less energy in this state and are more aware of what you need to improve either your stroke performance or your tactics.

Emotions can cloud your perceptions and your decisions. They can impair your timing and make you feel in a rush. Stay cool and eventually you'll play as well as you want.