Love of country. If your government orders you to fight, and threatens to hang or imprison you if you don't, that's pretty good motivation. If they toss on a cherry like "ending slavery", of course that's what you're gonna tell people you fought for. Of course "ending slavery" didn't become a goal of the war until about two years after it started.
Preserving the Union, and preserving the tax revenue from Southern states, are too nebulous, and are too distant, to create that kind of dedication.
For the common man certainly. For the movers and shakers that order them to do it, it was not nebulous at all. It was one of their prime concerns.
DiogenesLamp: "Love of country.
If your government orders you to fight, and threatens to hang or imprison you if you don't, that's pretty good motivation..."
"Love of country" is correct, the rest is nonsense.
Remember, the Fire-Eater Slave Power started declaring their secessions in December 1860, and immediately began provoking war with the Union by dozens of forceful seizures of major Federal properties (forts, ships, arsenals, mints, etc.), threats against Federal officials and firings on Union ships.
Each of these events was accompanied by waves of outrage in the North, and calls for military action against secessionists.
All such calls for military action were resisted by outgoing doughface Democrat President Buchanan, who was highly sympathetic to the Slave-Power and took no actions to stop them.
But the calls continued with each new Confederate outrage, and after Fort Sumter's surrender (April 14, 1861), President Lincoln responded by calling for 75,000 troops "to re-possess the forts, places, and property which have been seized from the Union."
But, as it happened, the very first Union troops mustered went directly to Washington, DC, to defend the capitol against the perceived threat of Confederates assaulting and seizing it.
By the time of the first Confederate soldier killed directly in battle, (Big Bethel, June 10, 1861) over a dozen Union troops had died, dozens more wounded and hundreds captured and held as POWs in Texas.
These all served to arouse Northern outrage against the Confederacy, and drove enlistments in Union militias.
But in early 1861, the existence of slavery in the South was not an issue Northerners cared much about.