There were many events in the 1850s that increased tensions between the two regions of the United States. Basically, those events all served to set North and South against one another. They caused the two regions to believe that they had different values and different interests.
Some of the events of the 1850s caused the two regions to feel that they had different values. These events reduced their feeling of being essentially similar to one another. Some examples of this were the Fugitive Slave Act, Preston Brooks’s attack on Charles Sumner, and the Dred Scott decision (the part of it which said that African Americans could never be citizens). None of these events tangibly hurt the North or the South. However, they played up differences in attitudes between the two.
Some of the events of the 1850s caused the two regions to feel that they had different tangible interests. The fight over the Compromise of 1850 (over which areas of the Mexican Cession would be free or slave) was like this. So was the fighting in “Bleeding Kansas.” In both cases, these were fights over political power in the country as a whole.
Both of these kinds of conflict led to increased feelings of tension between the two regions of the country as they came to feel that they had different values and different tangible interests.