Because, FRiend, most (not all) my Southern relatives' ancestors were Unionists, and to them you Southern flatlanders -- slave-owners or not -- were the "haughty bastards".
They didn't like you, and they didn't want any part of your stinking slavery, secession and war.
central_va: "A free rifle and ammo and authority to shoot Yankees is a tempting proposition for any Southerner.
IMO slavery was an issue drummed up as cause bellum out of convenience."
You forget that as late as 1856, Southern and Northern Democrats combined to win the election, which resulted in the Supreme Court's 7-2 Dred Scott decision, all but legalizing slavery in every state.
One or two more decisions like that would have made abolitionism unconstitutional.
And those were the stakes in 1860, but Southern Fire Eaters didn't want to play that political game, they wanted secession and they wanted war.
So they engineered it.
Now if you ask "why", the reason certainly had nothing to do with "haughty New England bastards", since they are not mentioned in any secessionist documents.
No, their reason was the obvious one, which they explained in great detail: to protect the future of their "peculiar institution", slavery.
You cannot understand, hating the Yankeeland isn't something we write down. It is just understood from birth. Like breathing.