Same old reason, greed...
The origins of the American Civil War lay in the complex issues of slavery, competing understandings of federalism, party politics, expansionism, sectionalism, economics, and modernization in the Antebellum Period. After the Mexican-American War, the issue of slavery in the new territories led to the Compromise of 1850. While the compromise averted an immediate political crisis, it did not permanently resolve the issue of the Slave power (the power of slaveholders to control the national government). Many Northerners, especially leaders of the new Republican Party, considered slavery a great national evil and believed that a small number of Southern owners of large plantations controlled the national government with the goal of spreading that evil. Southerners worried instead about the relative political decline of their region because the North was growing much faster in terms of population and industrial output.
The U.S. Constitution created a federal government with sufficient powers to both represent and unite the states, but did not supplant state governments. This federal arrangement, by which the central federal government exercises delegated power over some issues and the state governments exercise power over other issues, is one of the basic characteristics of the U.S. Constitution that checks governmental power. However, before the Civil war, it was flawed somewhat.
One major and continuous strain on the union, from roughly 1820 through the Civil War, was the issue of trade and tariffs. Heavily dependent upon trade, the almost entirely agricultural and export-oriented South imported most of its manufactured needs from Europe or obtained them from the North. The North, by contrast, had a growing domestic industrial economy that viewed foreign trade as competition. Trade barriers, especially protective tariffs, were viewed as harmful to the Southern economy, which depended on exports.
In 1828, the Congress passed protective tariffs to benefit trade in the northern states, but were detrimental to the South. Southerners vocally expressed their tariff opposition in documents such as the South Carolina Exposition and Protest in 1828, written in response to the "Tariff of Abominations". Exposition and Protest was the work of South Carolina senator John C. Calhoun, formerly an advocate of protective tariffs and internal improvements at federal expense.
South Carolina's Nullification Ordinance declared the tariff of 1828 and 1832 null and void within the state borders of South Carolina. It began the Nullification Crisis. Passed by a state convention on November 24, 1832, it led, on December 10, to President Andrew Jackson's proclamation against South Carolina, which sent a naval flotilla and a threat of sending government ground troops to enforce the tariffs.
It was this trade war, that fueled the division, and the final catylist was when another dispute over states' rights moved to the forefront. The issue of slavery polarized the union, with the principles espoused by Thomas Jefferson often being cited by both anti-slavery Northerners and secessionists on the debates that ultimately led to the American Civil War. Supporters of slavery often argued that one of the rights of the states was the protection of slave property wherever it went, a position endorsed by the Supreme Court in the 1857 Dred Scott decision. In contrast, opponents of slavery argued that the non-slave-states' rights were violated both by that decision and by the Fugitive Slave Law of 1850.