Why did Mozart write his requiem?

I was wondering why Mozart wrote his Requim. I knew he wrote it in Vienna in 1791 but I dont know why. I have read he started it for a countess, but I have also read he had written it for his wife. "commissioned the piece for a requiem m to commemorate the February 14 anniversary of his wife's death." Soooo why did he write it??? POINTS TO BEST!!!