Why did the soviet union want the us to open a 2nd front in the war?

The Soviet Union wanted the USA to open a second front in World War II because they were fighting a bloody, protracted battle with Germany on their home front in Eastern Europe.  The Soviets wanted a Western front reopened so that Germany could not focus all its efforts on its battles on Soviet soil.

Explanation:

In World War II, the Soviet Union bore the greatest cost in lives among any of the Allies fighting against Germany and Japan.  For the Soviet Union, the number of military personnel killed during the war was over 8 million, and perhaps as many as 10 million.   And the total number of deaths of Soviets, including civilians, totaled over 20 million -- perhaps as high as 26 million.  They had good reason to be seeking greater contribution from the Americans and other Western members of the Allies.

they wanted the u.s. to fight on the side of the central powers.