@aleo16, They only choose their own persona if they are "dumb" AI's. Cortana was one of the first "smart" AI's, which are created from an actual brain. "Smart" AI's inherit their mannerisms and personality from the brain of the person they are cloned from. An example would be that when Cortana told Halsey that she thought John (Master Chief) was cute when she was picking her spartan, Halsey blushed because she thought that too, and she couldn't deny it because Cortana reflected her opinions and personality.