Luffy thinks Zoro is fantastic! After all, the man who's going to be the greatest swordsman makes the perfect companion for the future pirate king! As a result, he has a lot of faith in him, and doesn't hesitate to trust him with jobs or requests that the others might complain about or refuse to do (like saving Smoker, going after Nami, or not fighting back when people simply aren't worth hitting). Knowing Zoro's involved in a situation tends to put him at ease (at least, more so than usual, since it's really hard to get him truly wadded up about anything in the first place).

He recognizes Zoro takes his promises seriously, too, so it's a common thread they share, which becomes important considering they've currently got, like, two promises going on between them. ♥ Luffy also realizes that he'll lose Zoro if he ever does anything that compromises himself as a captain or a man, so it's extra motivation for him to stay on track. If Zoro has any criticism or advice, Luffy will hear him out and usually act on it, even if it's stuff that's hard for him to hear. In that sense, Zoro's kind of an anchor that's consistently been around since the very beginning, and Luffy recognizes and appreciates that, I think, even if he doesn't really verbalize it. But, then, he and Zoro kind of have this thing going on where they really don't need to verbalize a whole lot, which is really nice.

Luffy also loves teasing Zoro for his crappy-ass sense of direction. I mean, HONESTLY. WHAT KIND OF PIRATE GETS LOST ALL THE TIME? GEEEEEZ. >:[

He might have flashes of insight where he thinks that maybe there could be something else going on too, but it's never really occurred to him to try to change anything between them, 'cause, well, things are pretty good as they are! But he's always willing to try new things, you know~ :D ♥

lol, I already wrote a whole essay about this at ship_manifesto, but I never get tired of talking about their dynamic. :B