If pressed, McCoy would barely admit to liking Jim at all. "What's to like?" he'd say, and then regale the interrogator with a list of James T. Kirk's many flaws and foibles. Alphabetically, if he were particularly irritated by the question.
He might point out that he'd had front seat tickets for the worst of it. Jim had latched onto McCoy as his preferred companion in crime -- or at least mischief -- and dragged him into scrapes that any sane person should have seen coming. That McCoy in fact often had seen coming, not that Jim listened. Which might cause his audience to wonder why McCoy let himself be dragged.
But if approached obliquely, applied with the right blend of mild interest and alcohol, McCoy would admit that he found Jim's unfounded, unsinkable optimism buoying. That he needed someone who still looked at an insurmountable problem and said, "I can do that."
And that maybe, just maybe, he found Jim's attitude contagious.