Why Cortana died in halo 4?

We know that the Master Chief and Cortana are more like lovers than copartner. What did the Master Chief look like under the halo 4 helmet when he knew Cortana has died?

Four years after the destruction of the Ark, when the Master Chief and Cortana awaked, they found they are being attacked by a splinter group Allianz. Caught in a good artificial gravity, they were down, with the remains of Forward Unto Dawn pulled the artificial planet Forerunner Requiem. To rotate after receiving a signal from the infinity of the UNSC ships, which received the signal of the dawn and finally captured well in gravity, they were the core of the planet, what appeared to be noise: however, on the contrary, they had an enemy warrior named Forerunner Didact. The Master Chief and Cortana narrowly escaped and fighted both Covenant soldiers and Amnesty International called Promethean they reach infinity. Meanwhile Cortana shows that it ever more prevalent, a condition in AI if their knowledge base was too large and gradually insane. When will thus destroy gravity called Forerunner awareness of the librarian tells him Didact: during the flood disaster, the Didact tried a device called the composer use (for the transformation of the life forms of digital creatures immune to infection, but created horrible abomination) of mankind, a race, which he hated because of the war between the races, in order to combat the tide in an army. He was of his own race was imprisoned with Forerunner soldiers and people; he had converted the Promethean, as his guards.

Contrary to the orders of Captain Infinity retirement on Cortana always unpredictable and escape chasing after Didact, he tried to deny, composer, hiding in an asteroid belt in the vicinity of one of the old halo rings. He can not stop the Didact, which reactivates the composer and turned people set up to study at the research station. The chef brings a nuclear weapon on board, while Cortana sanded spikes personality rampant in the system, disrupting the shield Didact. The Didact tried to kill the leader, but Cortana intervenes, allowing the head to defeat the Didact and triggered a nuclear bomb to save the destruction of the composer and the earth. Cortana performed a final act for the head, the protection of the explosion and left to save for UNSC ships: but while the Master Chief survived, Cortana died with the composer.

Cortana has died, and the halo 4 is end, I think it is reward to storage something about the halo 4 in my life. Maybe, the master chief helmet  is the best choose.