Japan invaded through Manchuria in WW2 so it makes sense for China to have as big a buffer zone as possible in that area.
The border between china and afghanistan is very mountainous and very far away from Beijing.
For example: one of the most criticized of China behavior, that is the occupation of many parts of mongolian and related lands, is a way to prevent another Khan starting another Yuan dynasty.
Similarly China had many wars with Korea, and don't trust them (neither do the Japanese, thus why they don't move a finger to get rid of North Korea either, nevermind blatant racism in Japan, like the fact that nonasians can expect to get citizenship after about 20 years trying, while many second and third generation Koreans still aren't allowed to be citizens)