The civil war was inevitable because we basically had two countries anyway. The North and the South had solidified into two different cultures with different needs. They were depending on one another, and they did not like it. There was also the issue of slavery, which allowed the North to feel culturally superior and the South did not appreciate that.
I think that you are going to find many different answers to this question. In my mind, the issue that gripped the nation in the decade plus leading into the Civil War was that of slavery. Both North and South had spent the first part of the 19th Century in developing their own means of economic growth and stability. For the North, this resided in the factory system and industrialization. For the South, this rested in the plantation system and slavery. To a great extent, both North and South viewed the other with a sense of misgiving because both systems were mutually exclusive to one another. The Southern plantation system would not work in the North and the factory system with its emphasis on crowded urban centers are something the South could not accept. In this growing schism, both sides viewed "the other" with skepticism and mistrust. In the decade leading to the Civil War, this intensified as one side felt that the other was wrong or at the very least, their side was more "right" than the other. Northerners featured a desire to maintain the industrial status quo, or went as far to advocate the abolition of slavery. Southerners believed in their traditions, of which slavery was an integral part and felt that Northerners were either as bad with factories in the North or intrusive on their own way of life. With such rooted intensities for both, conflict became inevitable. The 1850s were the period in which this was brought to the forefront of American society.
The Civil War may have been inevitable even before 1854, but a law that was passed in that year made the war even more likely to happen. That law was the Kansas-Nebraska Act.
Before the Kansas-Nebraska Act was passed, those two territories were going to be free territories. This had been decided by the Missouri Compromise of 1820. The Kansas-Nebraska Act reopened this issue. It allowed settlers in the territories to vote on whether they wanted slavery.
After this, violence broke out in Kansas between pro- and anti-slavery forces. This violence helped to turn the North and South even further against one another and made the war more likely to happen.