From the comments above, I reckon Master Chief will quite literally lose his mind at some point. Not so much break down but with anger, lose control of his emotion and within that moment he loses his focus on what his objective was and I guess shuts off his emotion altogether. Though, I'm not sure what he's going to do in Halo 5, I can see some interesting character aspect appearing from John 117, he needs some emotions to come through, it would give the game an interesting twist, well, mostly character wise. But, as I said he may even lose all emotion and become a machine... mentally and emotionally of course, the type of thinking that would make him shoot anything in site. Everyone thinks of John as a Spartan that can petty much get out of any situation, kill all enemies with no mercy and as one Spartan, one human... destroy all that is evil. He's a survivor, but now that Cortana is gone, the main A.I or his best friend (you could say) that was keeping him sane and human, is now gone. I'm curious to see what 343i will do, now that John is alone and with all he's been through, must suffer alone too.