Q. Why all the emphasis on rain in the prayers on Sh’mini Atzeret?
The officiant wears the white robe used on the High Holydays, the prayer is full of pathos and meaning, and it is clear that rain is a most serious subject of discussion with the Divine Creator.
In ancient times the lack of rain in the rainy season was an unmitigated disaster, which is why the Mishnah records the solemnity with which people would fast and pray in time of drought.
The first page of the Talmudic tractate Ta’anit notes that rain is one of three key things which remain in God’s possession; the others are the miracles of childbirth and the resurrection of the dead.
Praying for rain therefore acknowledges that there are some things which only God controls, and it is thanks to Him that we enjoy life and all its blessings.
For all that there is what the philosophers call the problem of evil (do we deserve the evil we see in the world?), there is also a problem of good (what have we done to merit so much beauty and blessing?).