Mill distinguishes happiness from contentment. (IP3 583b)

One is content to the extent that one does not have unsatisfied desires.

One is happy to the extent that one has pleasures and is not in pain. But not all pleasures count equal toward happiness. Pleasures of the higher faculties count much more toward happiness than sensuous pleasures since they have been shown to be of greater value.

Happiness, according to Mill, is "an existence exempt as far as possible from pain, and as rich as possible in enjoyments [pleasures], both in point of quantity and quality ..." (IP3 584b)