Probably the Bajorans are so freaking spiritual that a discussion about whether they wear their pagh (and hence their earring) on the left or right side is considered as banal and mundane as a discussion between guys about how they hang their member.
Yes, but where Bajorans hang their earring is out for all to see. Where a guy hangs his member is not. (Thanks for that image, BTW.) So all Bajorans dress right. Except for Ro. Why?
Well, that's not why. While we know the Pah-Wraith cultists wore their earrings on the right (when not incognito), they also wore special bright red earrings. Ro's are normal jewelry, presumably indicating her lineage (Ro) and her d'jarra (whatever that may be). Only Ro Laren, and, more importantly, all other female Bajora in the episode Ensign Ro, wear left-side, traditional earrings (d'ja pagh). What this indicates to me is that the Ros, and all other female refugees on Valo II adopted this practice for some reason. It could be a local eccentricity, a sign of distress (as stated above, vis a vis only female refugees at the end of the Occupation wear it) or a small cult/sect (maybe on basis of ancient belief) not seen elsewhere. It indicates a separation of gender, something not usually seen elsewhere amongst the Bajorans. It might be that Ro, and the other Valoans, are the ones who are orthodox, practicing an ancient form of prophet worship abandoned 5,000 years ago or somesuch, that was changed when they became more egalitarian (besides the d'jarras). That's just my speculation. Maybe we're supposed to ignore it, like the upper nose crease that all Bajorans decided to have surgically removed following the Occupation.
In show canon there is no story reason. In the novel canon, she wore it on the wrong ear to stop vedeks and others from grabbing that ear. She isn't very religious. The reason she kept wearing it altogether was in memory of her parents or something like that. From Memory-Alpha: So it sounds like it was something production decided on in TNG then later changed for DS9.
Thank you. Makes perfect sense. I was amused by Vedek Bariel's duck and weave to keep Winn from grabbing his ear.