Jim's mind is a treasure chest, a cave of wonders. It's something McCoy wishes he could take apart and study.
Jim has ideas, ones that hit at how smart he really is. Complex solutions with even thornier webs of calculations behind them. Jim never stops revising his strategy, improvising as unexpected variables are introduced. McCoy has no idea why Jim and Spock don't get along better; they have the exact same method of dealing as far as McCoy can tell. But Jim has something else, something more than Spock; that something is why one of them wears yellow and the other blue. It's the reason why, when their lives are on the line, he'll follow one without question and fight the other tooth-and-nail. Jim's ideas just seem to work.
Jim Kirk has never failed at anything he's truly put his mind to, as long as you define success a bit loosely. His confidence makes him eager like a flame, because everything in his way tends to be consumed and reduced to ash.
McCoy doesn't have the heart to inform him that not all of them are so lucky, so dangerously bright.
Jim thinks wild and bombastic things-- but they're not at all unreasonable because it's Jim who thinks them. As if he holds some magic behind those blue eyes to make anything seem within reach, even the stars. It's dangerous, it's pathetically easy to be caught in the eddy of Jim's enthusiasm. The path of least resistance, and after everything that's happened few of them have a lot of fight left in 'em. Barely enough to survive out here, let alone challenge their Captain.
Because everyone knows: if you stand toe-to-toe with Jim Kirk, you best be ready to fight with everything you are. Jim holds none of himself back, a dangerous tactic, one that's going to get him killed or worse, but it's just so damn risky that hardly anyone has the balls to ante up. Jim's all-in, every hand. After four years it never fails to put McCoy's heart in his throat.
(Jim is every metaphor McCoy's ever thought, more he hasn't yet, and still more he never will.)
Jim has these moments. McCoy hasn't named them yet, he's still thinking of something appropriately grandiose. In the meantime, he calls them Jim's soliloquies; they're usually when Jim's trying diplomacy rather than the big shiny guns he has attached to his ship. When he really gets going, Jim waxes poetic about the possibilities of the stars, of the unexplored regions of deepspace, how much potential it has and they have. How such an expansive universe could hold everyone, all types of races and cultures and beliefs. That each one was necessary, vital to making up the delicate web connecting them all for the benefit of all.
It makes whomever he's talking to feel like the most important person alive.
And there's never any telling how genuine he is, either, if he actually believes what he's saying or gives a damn at all. It takes all of five minutes to understand his easy charm is an act; what's below it is anyone's guess. There are days he hopes he has more of an understanding than the others. There are days he feels like the last to have figured it out. For all McCoy knows, Jim has been fucking with him this entire time. Unlikely, he decides, but still not out of the realm of possibility.
This is Jim Kirk, after all.
(Because somehow the name explains everything.)