Why would a japanese and russian guy fight in the 23th century?
Because Trek echos theories and ideas started during the cold war. It was controversial to have a Russian, Japanese & Black person all together in harmony on a TV show. Many viewers expected conflict, just as we do now with the fictious Maquis. However, we never got it in any case. Roddenberry made Trek to show that peace and harmony can happen between people of different religious, political and racial back grounds. He wanted Trek to be about over coming adversity thru tolerance and peace. Voyager continues the idea he started by not having internal conflict onboard Voyager.
Berman worked side by side with Roddenberry all thru TNG until he passed away. Roddenberry taught Berman just what he wanted his Trek to become. Berman followed and kept Voyager to as close as Roddenberry wanted his Trek to be. I believe we the fans (thanks to DS9) have forgotten what the purpose of Trek was to be. Roddenberry didn't create Trek to be dark & gritty, he made it to teach morality & promote harmony.