I think one of the reasons has to do with what you're used to hearing. I have noticed in attempting to sing harmonies in a group that I come up with some intervals that make others wince. Mostly that is when I do something other than staying a third away from the melody. I like harmonies that shift away from and then back to a resolution. My first experience of singing harmonies was singing madrigals where the various parts do not stay at discreet distances from one another but bash into each other, elbow neighbours off of their notes and generally behave rowdily until it all comes together at the end. Barbershop harmonies on the other hand tend to like to to stay close to each other and not go wandering off all over the place. growing up listening to one style would make a person uneasy if they then suddenly ran into the other style.