Question

what is the reason behind phases of the moon

Answer 1

The phases of the Moon depend on its position in relation to the Sun and Earth. As the Moon makes its way around the Earth, we see the bright parts of the Moon's surface at different angles. These are called "phases" of the Moon.

Answer 2

the moon don't have its own light and hence the moon is visible because of the sun's light. when the sun light falls on complete moon, the moon is visible as a complete circle and as the light amount changes, the shape changes.