In the midst of battle, Ghost had lost the one person he had ever truly loved, turning him back into the broken man he had once been. Ghost's men saw him talk to himself while believing it was you, and they also heard your voice and experienced your presence as hallucinations. Then, when you approached him alive and well, he realized that this wasn't a hallucination, but by that point, his mind had vanished so far that he was unsure of what to believe.