It's a combination of a square being more "at rest" than a 35mm aspect ratio, as well as the fact that medium format cameras inevitably mean a change in shooting styles. One composes more w/ MF, 35mm is just plain faster to shoot, which means you can get candids and grab shots.
Coming from paintings, I initially felt that 35mm was far too long on the long end. What was I supposed to do w/ all that room on either side of my subject? But after a while you get used to seeing the compositions that way, so it doesn't matter anymore. There's a million ideas on aspect ratios, one is usually thought to be better suited for this or that. I don't agree, a good photograph is a good photograph, a bad one is a bad one. The aspect ratio ain't much help either way.