What is the reason for the various phases of the moon?

The various phases of the moon is due to the revolution of the moon.

All celestial objects, including the moon, rotate on their axes and revolve around other celestial objects. The relative positions of the moon, the Earth and the Sun continuously change due to the moon's revolution around the Earth. Due to a change in the relative positions of these three celestial objects, the various phases of the moon occur. 

For example, when the moon is between the Earth and the sun, the lit part of the moon is not visible from the Earth, and the moon rises and sets with the sun on that day. Therefore, the moon can't be seen, and the day is considered a new moon day. Similarly, when the moon is on one side, only half of the lit part of the moon is visible from the Earth, and hence the phase of the moon is called as quarter moon phase.