How did the reliance on cotton production and slavery affect the South's relations with the North?
The South's reliance on these things affected its relations with the North in complex ways.
On the one hand, it actually tied the North and the South together. The South's economy was based so heavily on cotton that it needed many things from the North. Northern companies sold everything from shoes to insurance to Southern plantation owners. To this extent, the relations were at least somewhat good.
On the other hand, the reliance on slavery also led to the deep divisions between the two regions that caused the Civil War. This can be seen most clearly in the development of the abolitionist movement. Most Northerners were not abolitionist, but the issue of abolitionism did illustrate the ways in which slavery drove a wedge between the two regions.