She'd been by Master Chief's side for years. When no one else could understand what he was going through, or what it was like to be John-117, she was there to comfort and console him. It wasn't until Chief faced off against the seemingly unstoppable Didact that Cortana was faced with the one decision that will no doubt haunt John for the rest of his life. Without her help, Chief would have died at the hands of the Didact, but knowing she was deteriorating at an incredible rate, Cortana sacrificed herself to ensure Chief would live to see Earth once more. It was one of the more heartfelt moments in the series, and one we hope 343 Industries will stick to, other wise Cortana's death will have been meaningless.