Why is rain considered immortal in the poem 'Song of Rain'.

Rain is considered to be immortal because after it reaches the Earth it again evaporates and reaches the sky to form clouds and then again falls on the Earth. Rain has been personified here when it is said that it nourishes Mother Nature and then returns back to the sky. This is how rain has been considered to be immortal in the poem. She begins her journey with a thunder and ends with a vibrant rainbow.