The Doctor looked troubled. He was trying to imagine not needing the Master. But, he supposed, remembering everything he knew about the alternate time lines, if the Master had made him think he's left him, out of spite. When they were the last. That might do it.
"We fought in the war together. He carried me through it. Fought for his sanity to see us through. Defeated the drums. He's rational. Not exactly a paragon of virtue, but sane. And quite devoted to me." He said, bluntly.