Also, the stage play covers the suicide mission YorHa purposely sent A2's squad on. The same squad in which her girlfriend was a part of and A2 was the sole survivor. It basically explains why A2 is just so mad all the time. Also, explains the voices in the flashbacks she keeps having throughout the game. The one that tells her she has to stop crying and keep going is her.
Nier is a happy game about happy robots.