Why don’t we nuke North Korea ?

That would invite attacks from nk friends.

Leave other countries alone

You DO know there are no force fields, right?  So the irradiated dust would blow around the world and effect everybody, including in the US, "the only country that matters".

Because it would kill millions of people whose lives are already horrible and have no control over their own government.

You sound like Trump “if we have nukes, why can’t we use them?”

If we did that to NK, if enough of them survived they could try and nuke SK, or invade them. Anyways, with the amount of radiation put up into the air, it would be a very bad day for SK, Japan (both our allies) AND China. China would not like that very much. Hell, China would be able to detect our missiles and would assume they were inbound for Chinese territory. They would never tolerate the US taking out their friend NK or the US now being in a stronger position in the western pacific. SO...they would launch against the US and all US forces in the Pacific including hitting US forces based in Japanese territory and probably Hawaii, and of course California. The US would have to retaliate and hit China back. It would be VERY hard to keep from going all out after that. But let’s just assume we and China are able to stop there (which I doubt) so it’s a “small” nuclear war.  Yeah, millions dead, radiation everywhere killing millions across most of Asia including our allies, maybe only 10s of million Americans would be dead (if lucky) from actual nuke explosions and 10s of millions more dying from radiation, many US cities burned and glowing. Global agriculture takes a plunge starving millions across the whole globe.  

But hey, if we could keep it a small limited nuclear war, we would win, right? Hurray for us. Does that sound like a good win to you?

Because we are no longer the only nation that has nukes. If we start a nuclear war where will it end? Do you think Russia and China won't respond for their ally with nukes against us?  Do you think NK won't respond against our allies in their strike zone? If we were going to nuke NK, we should have done it back in the 1950s during the Korean war. Then maybe we should have gone into punish China for for their attack at the Yalu. That ship has sailed though, and we missed our chance. Too much time has passed. War is usually advocated by those who have never fought in one. If the US started the draft back up, and demanded 2 years minimum service from all Americans at age 18, would you still want to start a war up on foreign soil, if you might be sent in to hold the country? Probably, not. You would probably be carrying "stop the war" signs and flashing the "peace sign" and singing "kum by ya".

China says they're fighting against the side that fires the first strike of war.

Nuking N Korea, not only do we risk destroying people and the land for many innocent oppressed people where it isn't their fault that the regime is corrupt and they suffer from countless human rights violations, we also risk a war with China. That could cripple our economy. Because America's & China's economy is symbiotic. Just nuking N Korea right now would be negligent and extremely damaging for us. The US Gov's mission has always been rooting for the freedom of the oppressed N Koreans.

Why would we? Especially after Trump just spent four years fellating Kim Jong-un. That would be a terrible waste of a lot of mediocre fellatio. 

Because relatives in South Korea live there, don't need the Vietnam crazy Republican anti-commie sh*t again do we?.

The civilians need to be liberated, not killed.