We had been exploited by the same colonizer for hundreds years. The basic tactics for them to contaning each other is to bury hatre between us. If there was no McMahon line set by UK, where is that damn fcuking 1962 conflict coming from? If one sees a problem, you must see the root cause.
Look at this way.
Japan and China have a long history together.
How would you like it if the Japanese sent hundreds of well-trained and well armed militants/terrorists to a restive area of China every year to kill, maim and destroy? Their reasoning being that want to annex or 'liberate' that certain part of the country.
Obviously Japan is a civilised society so they don't do that sort of thing, but if you want to see this conflict from the Indian perspective, there it is.