Why did the union care so much when the south seceded? Why not just let them go and end up with a friendly border ally? I can understand instances where the portion of the seceding country contains an asset detrimental to the country's well being as a whole like an important port or rich farmland. But the US is giant even without the south and i can't think of anything the south had that couldn't be found elsewhere in the north.
Why was it so important to "preserve the union" during the US Civil War? Why did the union care so much when the south seceded? Why not just let them go and end up with a friendly border ally?
First, you're making an assumption that the Confederacy would turn out to be a "friendly border ally". There was no security in that, in either the long-run or even the short-run. Considering that Confederates opened fire on Fort Sumter while negotiations in Congress had not yet been exhausted (the Corwin Amendment had passed in Congress, after all, and was winding its way through the ratification process in the states, and peace efforts had been almost constantly underway since Election Day in November 1860), it would appear that the Confederacy from the outset was not particularly interested in "friendly" relations, unless the U.S. capitulated on all their demands they had failed to gain through peaceful negotiations in Congress. To the Confederates, "peace" and "friendship" would only come at the price of appeasement by the United States on virtually every political issue that had led to secession.
Variations on this question come up from time to time, and I have answered a couple of them. There were various reasons, which I go into detail in both the answers linked below. But the long and short of it was: there were a set of slavery-related issues that the two sides could not resolve in Congress, "just letting the South go" didn't do anything to resolve those issues, the South was determined to get a better "Confed-exit" through the barrel of a gun than had become apparent they could get through Congress or even through peaceful secession, and once the Confederates opened fire on Fort Sumter, the U.S. government had a choice to make about how to respond to this Confederate violence. The U.S. chose to respond with force, because they increasingly recognized that any "peace" was likely to be temporary if they didn't entirely submit to all of the Confederate demands, now and in the future. And the Confederate demands were steep.
Added to all that, the precedent of allowing secession as a resolution to a contentious but democratic political process, such as an election, was dire for the long-term prospects of the Constitutional government of the United States. After secession but even before the war broke out, there was already talk on the West Coast, and even in New England, of splitting off from the United States, too. There would be no United States - more like two, three, or four separate countries, with severe political differences, at a time when European superpowers could and had shown willingness to interfere, militarily, with North American politics. (France had been threatening war with Mexico since at least 1859, and proceeded with an invasion in late 1861. Canada was still a colony of Great Britain, with GB's engagement in the War of 1812 on the North American continent still in living memory of many older Americans, not to mention their military intervention to put down the Canadian Rebellions of 1837-38.) Splitting up into two or more separate countries was seen as inviting foreign entanglements, with each new country aligning with a different European superpower. This would mark the end of democratic government, and bring the former-U.S. back under monarchies and other undemocratic control again.
Keep in mind, too, that not all of the seceding states had seceded at the time the war broke out, yet they were still threatening secession - Arkansas and Virginia's secession conventions were even still in session. "Just letting the South go" meant that these states, or any state, could hold secession over the U.S. government's head as a coercive measure. If the U.S. government didn't bend to their political will, then they could threaten to leave, and join the Confederacy, or start their own new country all together. So, even without European interference, successful secession in response to a democratic election threatened to mark the end of democratic government.
This is exactly why Abraham Lincoln's Gettysburg Address is so famous, because it's essentially a succinct mission statement of why the U.S. was fighting, most particularly laid out in its first and last lines:
"Four score and seven years ago our fathers brought forth, upon this continent, a new nation, conceived in Liberty, and dedicated to the proposition that all men are created equal. Now we are engaged in a great civil war, testing whether that nation, or any nation so conceived, and so dedicated, can long endure...
"...this nation, under God, shall have a new birth of freedom and that government of the people, by the people, for the people, shall not perish from the earth."
The following longer answers to similar questions gives more specific reasoning, details, and sources on why the U.S. decided to fight to preserve the Union (the first one has the set of most commonly-cited Northern justifications highlighted in bold if you want a TL;DR):
The last time this was asked, /u/hillsonghoods also provided the following answer, with links to other answers, so I'll copy-and-paste it below, as these answers may also be of interest:
There's plenty of discussions of the causes of the American Civil War in our FAQ, and a more recent post on the topic by /u/freedmenspatrol, including this one which deals with the North's motivations, economic and not, and this one which discusses the general beliefs about slavery in the North.