Luffy says a lot of things without thinking; this, he does not.
It's not even words – not really. It's the surge of warmth and affection and the feeling that he gets when he thinks Zoro's name, all bright and there and perfect.
'I love you,' he thinks, but not in ways that make sense. He doesn't even ever recall thinking that, per say. It was more like a slow weaving of trust and adoration and admiration as they travelled together, and when Zoro lifted his sword to the sky and swore to become the strongest, even with the undercurrent of worry he felt at seeing that much blood, Luffy couldn't help but grin and laugh – because of course he would.
Of course he would.
Besides, he shows better than he says. Quiet moments in the span of seconds, pressed hands and knuckles bumping, a fraction of time when his gaze lingers just longer on Zoro and his grin stretches just a bit wider, the turn of his body to accommodate the person he knows will always guard it for him, because this is how he shows his trust (I know you have my back).
And he shows it a lot, in ways that don't make sense (to anyone who doesn't know him, at least.) It is careless laughter and the knowledge that if he falls in the sea he will always have someone to save him, and more often than not, that person is Zoro; beating heart pressed against Luffy's arm as he swims them back to shore or hauls the both of them out of the water.
He's perfectly fine with that not being 'love' to anyone who thinks they know what it means; because this is fine by him, and when Zoro accommodates him – shifts his body so Luffy can lay his head on Zoro's knee and nap, draws his swords against slights he knows Luffy won't take, fights for the dreams of all of Luffy's crew (their crew, even though Sanji and him fight like cats and he calls Nami a witch and grumbles because Robin always has her secretive smile and Franky is often so loud and Usopp can be a wimp and Chopper just puts on way-to-many bandages,) he knows it's fine by his swordsman, as well.)
So he does not need to say I love you, because he knows that Zoro already knows.
AN: A birthday present for a friend from November that I may have forgotten to upload? /oops