China was forced to give up enclaves in Shanghai, Hong Kong and Harbin in the 1800's and today's leaders consider that to be a national humiliation that they will not allow to be repeated.
They consider Tibet to be a backward, primitive society with lots of resources to mine and lots of room to settle Han Chinese in.
They believe that they are bringing modernity, health services, roads and bridges to the Tibetans, which they are.
Tibetans claim they are losing their traditional way of life, which is also true.
The Chinese have treated the Tibetans better on average than the Americans treated the Indians, like the Sioux and the Navajo.