As a northerner, it always amazes me how modern unionists are so angry at the south. Truly sore winners.
I think that's only part of it. "Waving the bloody shirt" has long been a useful political tactic, and I think it is politically advantageous to some extent to constantly bash the South, especially when you are courting Liberal voters.
I also think some of it is just "Rah Rah team!" When people live in states that are on one side of the conflict or the other, they adopt those state histories as their own, and they cheer their states past as they would a sports team.
Also, a lot of people want to be on the side of the winners, and of course there is the ever present propaganda on the topic which states the war was fought entirely over slavery, with the good side fighting the evil side.
People want to be on the "good" side.