The Gettysburg Address summarized what the war was about, IMO. What caused it is a different question.
On the analytical surface, it was the secession of the lower southern states, caused by the election of Abraham Lincoln, caused by the Free Soil movement that voted Republican. *
Free Soil is the doctrine that stipulates it's proscribed to bring slaves into the territories and to create new slave states.
Why this mattered to Free Soilers was as much about slavery being a threat to their own freedom as it was for compassion for the slaves themselves. Probably more so.
Anyway, there were enough people on both sides who decided that this issue--slavery in the territories--was worth fighting and dying for. There were already violent regional conflicts in Kansas, Nebraska and Missouri in the 1850s over this issue.
After Lincoln's election, both sides felt that the triumph of the other side would wound or even terminate their own sense of being an American. Each harbored, according to the other, irresolvable mutually contradictory ideals. The fragility of the republic cannot be underestimated, and neither can its "fluxation", which, at once, was a virtue and a danger to Americans throughout its history to that point. Americans were ever optimistic about the future and also wary of it, given that hyper-vigilance was required to maintain it. Also, they were looking over their shoulder at thousands of years of oppressive government as the norm, and the Experiment as the incredibly improbable exception.
So, when slavery and its effects are both considered American and Un-American at the same time by opposing sides and each believes the other is willfully deceiving themselves in an effort to actively subvert American ideals in a desired exchange for tyranny, that's a boiling pot for conflict.
*With grateful acknowledgement to J. Mills Thornton III, professor Emeritus, University of Michigan, as expressed in his course The Ordeal of the Union.