Here a info from the newsgroup alt.fan.sailor-moon thanks for reading,
> "Chris Kern" <chriskern99@...> a écrit dans le message de news:
>> On Tue, 29 Aug 2006 20:17:54 -0400, "Steven Yoruno"
>> <sailor_night@...> posted the following:
>> It's just an inside joke; the animators seem to have (for whatever
>> reason) put Ami and Makoto together a lot (maybe just an inside joke
>> among the animators, or maybe just laziness).
> I guess is more laziness, they could had lost of opportunities to put Amy
> with Raye/Mina/Michelle(Michiru) or even Hotaru (seems then some
> doujinshis authors didn't lost this opportunity however ^_-)