What did I miss? What did Afghanistan do to upset Pakistan so much?

They host and train terrorist groups doing attacks against Pakistan. There's even a Pakistani offshoot of the Taliban.

Oh my. Thanks for the info.

The Taliban literally attacked Pakistan on the day they started ruling.

Once they've driven the country back to the stone age, it will get divided up between Pakistan and Iran.

It's why Pakistan also supported the American occupation despite their population very much being against it.

The Taliban are militant extremists even by the standards of a state that incorporates literal Sharia law, with a theocratic court than can declare any law void for conflicting with Islamic law.

Of course, Pakistan supporting the occupation is also why the modern Taliban has beef with them so... War, change, not doing that, etc.