It's the internet. When you talk to people online, it often descends into pettiness. When you talk to people in the real world, that rarely happens. But it's much easier to talk online, so people get the wrong impression. You should talk to strangers. It's never gone wrong for me. Most people have a warmth and agreeableness that comes out when you are there with them, talking about stuff. There's also the interesting…
My life experience differs: I, for example, am likely more disagreeable in real life than on the internet. :-)
In internet forums, all answers are "delayed" (since you have to put them into a coherent text). Thus you rather have to think through your arguments, and thus react less on impulse (the impulse is typically already over when you have finalized your post). On the other hand, in real life, things that you strongly disagree with might trigger a very spontaneous emotional reaction.
Also, in internet forums, you want to "win internet points". Thus, you only tangentially write your arguments for the counterpart (who you will likely not convince), but more importantly to convince the many people who read your post. So your arguments are better well-thought and well-researched.
In real life, there is often no additional audience to appreciate and be convinced by "your great arguments" ;-) so it's an "all-or-nothing" to bring the truth to this ignorant being. Since the counterpart has such as "stupid" opinion, rational arguments are likely not the best way for this (because if the counterpart was capable of rational thinking, they would immediately see how "stupid" their opinion is ;-) ), so you better "hammer" your arguments into the counterpart. :-)