I genuinly don't understand why North Korea and China have such a... "good"?... relationship. Since this seems like an open secret, idk it just boggles my mind

I genuinly don't understand why North Korea and China have such a... "good"?... relationship. Since this seems like an open secret, idk it just boggles my mind. Like I constantly see articles about it happening, but I have yet to see anything about why it is the way it is. Obviously cheap labor is one thing, but I wouldn't think even China can ignore the threat of North Korea working on missiles? It also makes me wonder, if the people writing these comments know that they are working with people from North Korea or if they just assume its another company.