The south was fighting to expand slavery which the North opposed. The North was fighting to preserve the Union which the South didn't care about.
You do know that slavery was legal in Massachusetts until after the War of Aggression, don’t you? In fact, it was illegal to free your slave in Massachusetts if you didn’t post a bond to do so. Boston was 10% black at one time before the War of Aggression.
The reason I think the South cared about the Union is because a Southerner, George Washington, was the Father of the Country. And because a Southerner, Thomas Jefferson, is credited with writing the Declaration of Independence. And because a Southerner, James Madison, was the Father of the Constitution. And because a Southerner, James Mason, was the Father of the Bill of Rights. Southerners loved the Constitution, the country, and the Union which they “helped” found. They were willing to fight for their Constitutional rights. They did not love the synthetic Constitution evolving in the north which they feared would be used to destroy the South politically, economically, and militarily. (We now know the fear was justified).
You are bold to assert the North was fighting to preserve the Union and not for some high moral cause. You wouldn't believe how many people believe Julia Ward Howe's emotional song (”As he died to make men holy, let us die to make men free”) represented the purpose of Lincoln's optional war.