Cortana has always been one of the most fascinating parts of the Halo fiction for me. A.I.s in the Halo universe are just very interesting in general; they're based on the neural pathways of a human brain, which means their initial intelligence is that of a human, but they can perform far more simultaneous tasks and acquire more knowledge than any one human ever could. This comes at a price, however, with one of the core conceits of their existence being that they only have a lifespan of around seven years. These so-called "smart A.I.” (there are “dumb” A.I., too, who don't deteriorate) acquire so much information that eventually they just cannot process it all and quite literally will think themselves to death. This is known as Rampancy, and often sees the A.I. turn against their masters as they begun to fully comprehend the finite nature of their existence.