Lincoln went to war because the economics of his situation required it. So long as Washington D.C. controlled the South's economic policy, it could glean off profits for Northern Industry which would not otherwise be possible. More importantly, it could prevent the creation of competing Industries, and eventual competing economic interests in the Mid West from the capitalization which would have occurred in the South with direct European trade with the South.
In Simple terms, an independent South was a serious financial threat to the Crony Capitalists of the North and their agents in Washington DC.
Look up "Mercantilism". That was Lincolns political Philosophy, and one which he acquired from his mentor Henry Clay.