Planets have moons because early in their formation they were introduced to other space- faring rocks that either crashed into the planet and threw off debris, or were trapped in the gravitational pull of the planet. Over billions of years the orbits of these rocks (or debris), now under the inﬂuence of the planet’s gravity, were squashed into a spherical shape that kept them encircling their host planet.
To visualise how a moon becomes ensnared, imagine the earth is a ball placed on a ﬂoating sheet of frictionless paper (to represent gravity). The ball depresses the paper and if you roll a coin (to represent a moon) around this depressed area at a fast enough speed, it will circle the ball indeﬁnitely.
So, why don’t moons have their own moons? The answer is that the planet they are orbiting has a much stronger gravitational pull, so over time it would selﬁshly take other objects in as another moon for itself.