Why is China so worried about Tibet?

Commie trolls talk about serfdom on Tibet, but they don't mention that Tibetans are serfs to China. Ever since the invasion of China in Tibet, Tibetans are not allowed to choose their leader, Tibetans are not higher in society than any Chinese, Tibetans are forced to learn Mandarin, they are even required to show their ID before shops will allow photocopying services to them.