People simply don't like to be told they're wrong, regardless of how nice you are. Effectively receiving criticism is a learned behavior. This is basic human nature, and you will encounter it in any space, be it work, school or a video game like FFXIV.

The FFXIV community is weird in comparison to other games though, in that here we blame the person reaching out and giving the advice rather than the person who becomes aggressive after receiving it. This is extremely disheartening to many people who are genuinely trying to reach out and help other players; I've seen many, many chatlogs and cases where someone is being as nice as humanly possible while giving advice, and the other person still launches into a tirade and bitterly attacks them. I've even seen it happen in settings where people willingly submitted themselves for criticism. I know a lot of FFXIV educators that have quit teaching entirely because they encountered one too many cases of this kind of thing.

Personally, I switched from teaching pug learning parties to taking on individual, dedicated learners. I still get the occasional crazy, but the environment is much more controlled and all of the communication takes place on a third-party platform. Improved my teaching experience ten-fold.