I feel like this cannot be stressed enough:
High Elven and Human history and culture are deeply interwoven. Humans, Dwarves, Gnomes, and Elves. They’ve co-existed for such a long time, they are dependant on each other, living in symbiosis more or less.
One kingdom can’t hope to produce all the goods they require to maintain their way of life. From food to clothing, metals, stonework, and a hundred other things.
They enjoy largely the same things, like music, art, theatre. Their values are similar, and they look rather alike in many ways.
One of Quel’Thalas’ most famous and influential families is pro-Alliance, the Windrunners, the eldest among them being married to a Human Paladin.
They fought together in the Troll Wars and the Second War. They’ve adventured together into the lands of the Eastern Kingdoms, founding lodges, and fighting off trolls and ogres.
They taught each other magic, and how to wield the Light. Worshiping the same Holy energy through ritual and doctrine for aeons.
They’ve married into each others’ families, to the point where an untold number of High Elves sailed across the sea with the Arathi to form a new empire.
Yet some players feel it makes more sense for these High Elves to join a horde of alien invaders, bringing with them demons from the Twisting Nether, raising the dead into service, enslaving dragons, and burning the Eastern Kingdoms to ashes in a frenzy.
…
It’s pretty much as if Klingons invaded Earth, burning France to the ground, and spreading their fel and necrotic magic everywhere. One of their chieftains is even installed as a lord of death on the North Pole.
Then a few years later, the Prince of Denmark got corrupted by the fel and undead energies brought by these invading aliens to Earth, and so he first destroys his own kingdom of Denmark from within, before invading the Netherlands with his army of the dead, burning it to the ground.
Then the Klingons who started all of this decide to settle down on Earth, and some of their children calm down a bit, no longer intent on conquering the entire planet, just surviving.
A while later, a racist comes to power in the remnants of Denmark. As the survivors of the Netherlands seek help in re-building their fallen kingdom, he treats them horribly, caring only for the wellbeing of his own citizens.
Meanwhile, however, other human nations stand strong. France has been re-built and is stronger than ever. Some dutchmen even live there.
Now, does it make the most sense for the desperate survivors of the Netherlands to seek out the Klingons and form a pact with them, forever becoming enemies of all the other human nations on Earth, cut off from trade of goods they desperately need… or does it make the most sense for these Dutch survivors to seek out France, Belgium, Norway, Sweden, etc., and enter a new Alliance with them?