Today I noticed this post on Reddit, and found a discussion people were having about if Locke could beat the Chief in a fight, which in itself probably deserves its own thread, but the thing that struck me in particular was how people were referencing Chief's "current mental state" as a weakness he would have.

I know i343 is trying to portray the loss of Cortana as a big hit to the Chief's emotions, but has there been any discussion (which I'm sure there has) on why the loss of Cortana would impact his performance in such a major way, while losing several of his fellow Spartans doesn't seem to have affected his performance in a noticeable way during the events of Halo 1 through 3?

Is the reasoning that he cannot function properly without Cortana? Which he seemed to do fine during the events of Halo 3?

Is it the fact that it was his mission to protect her and he failed? In that case, wouldn't coping with a failed mission be a part of a Spartan's training? Or is it that they can cope with the loss of a fellow Spartan because they know that's a cost of some missions, but not losing Cortana WAS the mission, and they never trained for that?

Or does it simply come to down to the new version of the Chief i343 is trying to portray?