Though I’ve now lived out of the south for longer than I lived in it, I’m from Mississippi and still feel like a Southerner.
For me, it’s the atmosphere. Some places are southern simply because they aren’t anything else. West Virginia is a great example: what could it be other than southern? It’s not the northeast, though it borders it. It’s not the midwest, though it borders that, as well. The accent is right, the cuisine is right, the music is right, it’s the south, darnit. (Even in the northern panhandle. 20 minutes outside of Pittsburgh, right over the border, there are places where you may as well be in Alabama I tell you!)
Missouri and Kentucky are the same way, there’s just no other way to describe them.
Maryland? Well, despite it’s position below the Mason-Dixon line, no, mainly because no self-respecting southern state would elect a Kennedy to their state executive.
But some states are weird, they have to be divided up, and that’s bizarre but again, it’s about what makes sense. Northern Virginia, up in the DC area? Absolutely not southern, but you can’t say that Roanoke or Richmond, the capital of the Confederacy, isn’t the south. Florida, beyond the panhandle, no. Texas, certainly, so long as you’re talking about rural, eastern Texas, not the cities and not the west or the panhandle, which are decidedly “southwest” like New Mexico and “west” like Oklahoma, respectively.
The South: It’s A State of Mind.