It took a very long time for the sections to reconcile after the Civil War. In some ways, they still have not. To the degree that they have reconciled, however, it has been because they have become much more like one another economically, politically, and even culturally.
This all started with the movement of Northerners to the South after WWII. This movement, which has sped up in recent times, made the two sections more culturally similar. The people came largely for economic reasons, drawn by job opportunities in the South. The South has attracted a great deal of new business, making it economically more similar to the North. Finally, the South has stopped being “solid” for the Democratic Party. It is now much more Republican, linking it to states in the Midwest and making it much less unique in that sense.
In these ways, the South has become more like the North, causing sectional tensions to ease.