Dear Rinchen,
There's a whole history behind Arunachal Pradesh which is also known as the North-East Frontier Agency (NEFA). You can read more about this disputed territory here:
https://www.wikiwand.com/en/Arunachal_Pradesh#/Sino-Indian_WarTo put it simply, the British colonizers in 1913–1914 called on the representatives of China and Tibet to meet in India to define the borders between Inner and Outer Tibet as well as between Outer Tibet and British India.
The British came up with the 550 miles (890 km) McMahon Line as the border between British India and Outer Tibet but the Chinese representative refused to accept the agreement and apparently walked out.
But the Tibetan and British governments went ahead with the Simla Agreement and declared that the benefits of other articles of this treaty would not be bestowed on China as long as it stays out of the purview.
The Chinese position was that Tibet was not independent from China, hence Tibet could not have independently signed treaties. And per the Anglo-Chinese (1906) and Anglo-Russian (1907) conventions, any such agreement was invalid without Chinese assent.
The NEFA (North-East Frontier Agency) was created in 1954 following the boundaries established by the McMahon Line and the issue was quiet for nearly a decade, a period of cordial Sino-Indian relations, but the re-emergence of the issue was a major cause of the Sino-Indian War of 1962.
During the war in 1962, the PRC captured most area of Arunachal Pradesh. However, China soon declared victory, voluntarily withdrew back to the McMahon Line and returned Indian prisoners of war in 1963. The war resulted in the termination of barter trade with Tibet, although in 2007 the state government has shown signs to resume barter trade with Tibet.
Hence HH Dalai Lama visiting Arunachal Pradesh brings back the whole history of this disputed political boundary which China has never accepted.
It is also a challenge to China's assertion that Tibet was placed under the sovereignty of China when the Qing dynasty (1644–1912) ended the brief Nepalese rule (1788-1792) from parts of Tibet in c. 1793.