(Lower your voice for commands and raise it for praise.)
Why is this?
Pick up two glasses of different size from your kitchen and lightly strike a spoon against their sides. The bigger glass will give off a lower sound than the smaller glass. Bigger objects/animals give off lower/deeper sounds than smaller ones. Your dog doesn't know you are lowering your voice by choice; they only know you "seem" larger when your voice is lower...which makes it more likely they will listen.