Nationalist pride, sure, that's been a constant through any cohesive nation's history. Koreans aren't Chinese, though, and the leaders of China are too smart to look for a "short, victorious war" on the Korean peninsula..
They don't even have an idealogical interest -- we throw around the word "communist", but North Korea's modern ideologies are so far removed from Chinese communism that it's practically apples and oranges.
And the Cold War is long over, nobody's out to "eradicate" or even "contain" communism. US-Chinese relations are strained, but not hostile, and they know we wouldn't be looking to cross the Yalu.
In the late 1970s, Deng Xiaopeng killed off the last group of leaders trying to keep that sentiment alive (the Gang of Four) and since then they've been all pragmatism, all the time. Any talk of communism is window-dressing, their regime is ideologically committed to doing what's practical.
So even aside from the cost-benefit, they have no ideological incentive to throw their lot in with the North Korean gov't. As I said upthread, the reason they give them aid right now is because a refugee crisis in Manchuria would be inconvenient. Pragmatism.
