I'd actually say that Majora's Mask is about life.
Or rather, not letting petty squabbles, past grudges, or even regret keep you from enjoying life until the end. (Or in two cases, prevent you from moving on to the afterlife. /spacewhaleaesop)
Grief, pain, sorrow, happiness, longing... these are all a part of life, but we shouldn't let any one of these stop us from living, even in the face of imminent death.