Why is it so easy, and so tempting, to be rude online? Generally I think I try to be pretty polite but sometimes I post responses to threads that make me cringe when I read them later, things that make me sound like a petulant child. I don't say things like that in 'real life'. Something about the difference between talking to someone in person and posting on the internet makes it too easy to be snarky or over the top in responses, that's how it seems to me anyway.
To echo what others have written, a lack of substantive consequences is definitely a factor, especially on sites where people are able to use aliases and never reveal their actual names. Most typically all that happens for petulant childish behavior or snark is to be given the online version of a time out or grounding. If matters escalate they can then be banned. But if being banned from an online community has any significant impact on one's life it may be to their own benefit that they are banned. Coming here for fellowship can be meaningful but it shouldn't ever be a substitute for social structure in real life. I sometimes worry when I see people in their post-youth / middle-age years, not young enough to still be in school or old enough to be retired, who seem to post all day long.
Ideally, people come onto forums like this one when they're finished with all they actually need to get done, so maybe their fuse isn't as long as it had been at the start of the day, and their brain isn't as sharp. Aggressions or frustrations of the day can be taken out online, tiredness can show. It's like how sometimes people who are known for being polite in person may honk more obnoxiously than necessary if stuck in traffic at the end of a trying or especially long day, or if hurried in the start of it. When behind a wheel on a freeway there's definitely more repercussions for serious misconduct than on a website, but more minor things like simply being a jerk and not letting the person in the lane next to you in, or honking at the car in front of you the second the light turns red to get a move on, there's less accountability than for the equivalant of that behavior at work or school. Similar for here.
I usually write my posts either when I'm stuck in the car in traffic (I don't drive, so I'm not posing a hazard, haha) or when I can't sleep / am procrastinating sleeping, or during times of stress, as a diversion. It's a way for me to wind my mind down or refocus it. It's why my posts are sometimes ridiculously long.
I'm often no longer using a filter on my brain, I'm just dumping out all thoughts stuck in there.
I do have several friends from my personal life who are members here, but the majority on CF are strangers to me. I know them by their user names and the pictures they choose to represent themselves. It's very one-dimensional. We're reacting to written words, without the added benefit of tone, body expressions, or genuine familiarity. We're from very different backgrounds and ages. That was one of the appeals to me when I joined here in HS. On social media you primarily interact with your peers, family, friends of your family, neighbors. It's rarer to regularly interact with someone 50 years older than you who lives someplace you've never been and holds beliefs and views that are so unlike your own. I think it can be expansive in a healthy way, if you let it, but also be the cause of frustrations.