Why did Northern men volunteer to fight in the Union Army?

To preserve the Union.

Treason is about as good a reason as any.

They fought to have all of the Confederate statues removed from Southern cities.

The draft came later in the war.

This quote from Ang Lee's "Ride with the Devil" just about sums it up for me.

Mr. Evans: They (Yankees) rounded every pup up into that schoolhouse because they fancied that everyone should think and talk the same free-thinkin' way they do with no regard to station, custom, propriety. And that is why they will win. Because they believe everyone should live and think just like them. And we shall lose because we don't care one way or another how they live. We just worry about ourselves.

Jack Bull Chiles: Are you sayin', sir, that we fight for nothin'?

Mr. Evans: Far from it, Mr. Chiles. You fight for everything that we ever had, as did my son. It's just that... we don't have it anymore.

Because their factories would have been empty if they couldn't force the South to sell their raw materials to the North rather than France and England -- using their numbers advantage in Congress to impose export tariffs.

using the same logic, why would americans volunteer to fight in wwi, vietnam, korea?

Go and watch Ken Burns' "The Civil War".
One of the few good ones he made.

Because farming is boring and tedious.

quote:

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There was a draft but my understanding is that drafted numbers were relatively low.

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At the beginning of the rebellion, the Union took a number of 3 year men as volunteers. The secesh enlisted the bulk of its army for a year. Figures are hard to come by because the secesh forcibly reenlisted many who don't show as drafted. About 1/3 of the rebel army was drafted one way or the other. About 6% of the 2 million Union soldiers were drafted.

After a year a lot of the rebs had had enough soldiering. They were forcibly reenlisted or shamed into avoiding the draft the so-called CSA instituted in 1862.

Some large number of Union men, despite the maladroit management of affairs, especially relating to the Army of the Potomac reenlisted in the summer of 1864, despite the brutal campaign they knew that Grant would pursue.

They enlisted and reenlisted because they knew that there could be no appeal from the ballot to the bullet.

"Peace does not appear so distant as it did. I hope it will come soon, and come to stay; and so come as to be worth the keeping in all future time. It will then have been proved that, among free men, there can be no successful appeal from the ballot to the bullet; and that they who take such appeal are sure to lose their case, and pay the cost. And then, there will be some black men who can remember that, with silent tongue, and clenched teeth, and steady eye, and well-poised bayonnet, they have helped mankind on to this great consummation; while, I fear, there will be some white ones, unable to forget that, with malignant heart, and deceitful speech, they strove to hinder it.

Still, let us not be over-sanguine of a speedy final triumph. Let us be quite sober. Let us diligently apply the means, never doubting that a just God, in his own good time, will give us the rightful result."

Yours very truly
A. Lincoln

-8/26/63