I think anger and fear are for losers and for the weak. These two emotions don't fit with SIth "survival of the strongest" behaviour and their winner mentality. Anger greatly diminishes ones focus and mental performance and consequently increases chance for errors. Winners have no reason to be angry while losers do. As for fear, fear is a powerful weapon so therefore the Sith should induce fear and terror into their enemies, not the other way around. A Sith who is afraid sounds pathetic and weak.
The animal kingdom's always been that way. Fight or flight. The Sith personify the survival of the fittest, and the show it by the predominance of their emotions. Sith Sith aren't angry that they're winners. They're angry that the losers dare to still exist. the Sith channel their fear for power, since fear is one of the most easily accessed emotions we have.
It actually fits quite perfectly.