Why is China so obsessed with Korea?

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It's scary once you start digging into their history and the Northeast Project.
They are already defining Goguryeo as part of Chinese history;
but it won't stop there. They will likely claim Baekje, Silla, and Gojoseon are all part of Chinese history as well.
They are a country that already claims Joseon was founded by Chinese people, citing the "Gija Joseon" theory.
What is truly terrifying about Sinocentrism is that
many Han Chinese view Korea as a sort of autonomous vassal state that they once ruled.
This isn't just limited to the mainland; those in Hong Kong and Taiwan who are steeped in Sinocentrism are the same, because they are Han Chinese.
Since long ago,
China's own security has fluctuated
depending on how much influence they can project onto the Korean Peninsula.
That is why China decided to launch an expedition during the Toyotomi invasions.
Even now,
some North Korea experts argue that
if the North Korean regime faces a crisis,
the Chinese military will immediately push tanks from the Yalu River toward Sinuiju to take control of North Korea.