MugiAzusa > MugiRitsu
Azusa seems to often be in awe of Mugi, both in the manga and the anime. I even highlighted as much in one of my episode impressions, and it's something that was overt in its presentation. Azusa always sees the rest of the group as a curiousity, particuarly Mugi and Yui, who often satiate that need.
But Mugi being such an affable character, these sort of things tend to come easily. And it's easy to see why these pairings are made.
Summary: MugiEveryone