Who or what is the subject in Velazquez's Las Meninas?

In the painting Las Meninas. The subject in the painting are the King and Queen. As he prepares his brush for the canopy Velazquez takes a glance at the subjects but then he decides to paint the subjects in view of the King and Queen instead--for some reason he decides to paint the action in the room, even the dog. It seems to me he found this more interesting than the royal couple.