I think it mostly has to do with other people's insecurities. If I say something like, "veganism is unhealthy" (because of lack of B12... yes I'm aware there are nuances), some vegans interpret that as a statement of opinion, rather than a statement of fact, or even as an attack on their lifestyle choices, because they are so involved with their egos.
I try to follow Paul Graham's dictum "keep your identity small", the saying "strong opinions weakly held" and/or I've extremely strong self-confidence (compared to most people), so I seem to be way less personally affected by statements like these (though I'm sure I have other triggers)...
Granted I'm only 30 so maybe that will change in a few years, but for the time being I've started to moderate my stated opinions to spare other people's feelings (though I still think it would be better if they grew more emotionally mature, but I realize that's unlikely to happen), to improve social relations and, frankly, also because it's getting more and more dangerous out there, with political correctness and/or China-driven character assassinations.