Hamkawa!!

The first reason I came up with the idea that humans should be separated was because of their observed behavior around each other. Humans are highly intelligent, and like most intelligent animals (like otters or dolphins) they use this intellect for self-gratification or violence, whether it's 1v1 or on mass, someone somewhere is either attacking someone or wringing someone out until they have nothing to offer, even if they don't want to participate.

Just look at the news aligned to any bias or any popular prank "trend". There are bad people on all sides.

Ever wondered why every single fandom has a "loud minority"? Furthermore, why is it that whenever there are more people in a group (religion, fandom, and even political groups) it seemingly becomes more hostile to anyone outside its group or anyone who doesn't follow their normalcies? It's not a coincidence (nor is it minor, if there are fewer "toxic" people than them then why are the nicer ones the supposed "silent ones").
It's inevitable human nature.

You see we evolved from a species of apes called chimpanzees, these animals are extremely violent, especially when compared to other primates like the orangutan (who are very peaceful despite their strength). A specific behavior of the chimpanzee that I would like to point towards, is the fact that they actively hunt down the young of other primates. This is most likely to get rid of competition or to consume as much as they can (which I wouldn't doubt since they're animals who aren't very aware of the whole "consequences" thing") and may have carried over into us. Behaviors become culture, and culture slowly becomes ingrained over centuries or millennia. After all, we never stopped having wars or targeting others for being different (even if it's slight or harmless) have we?

What I'm trying to say is that the desire to or action of harming, hating, and irritating anyone who is slightly different from us may be ingrained into our blood. Our domestication of ourselves and mild cooperation with each other just aren't enough to make for the violence. It just covers it with a tiny bit of sunshine and rainbows (the same can ESPECIALLY be said for entertainment).

That's the problem with humans being social creatures. The human race is overly indulgent in desires, regardless of whether it is good for the individual and others, or not. If we rely on each other to make ourselves happy then we can end up stressing those who don't want to be involved or leave them be and stress ourselves if there are no other options. We need to strive to be self-reliant as a species, otherwise (this with overpopulation) can cause the same effects as the rat utopia experiments, exhausting and stressing eachother to the point of mass suffering and extinction.

Overindulgence also affects the landscapes around us, turning peaceful villages, beautiful forests, and even diverse and well-balanced cities into war zones, toxic areas, and cold dystopias.

Humans are also very slightly different from each other, from birth to memories and interests. In a way, it seems like we're completely incompatible with each other, yet we are still naturally social animals despite the risks. We are our own worst enemies. If we want to keep our individuality without being punished, we should stay away from each other and quell our social instincts, even if it's just a little bit for an elongated timespan, so we can eventually grow out of it or at least wait until the war is over and better manage ourselves.