Why did the Chinese develop a sense of superiority?

Answer 1

Because for thousands of years it was considered a leading civilization. Civilization in China is meant to have developed thousands of years earlier than western civilizations. The view of foreigners by the Chinese civilization was that they were all barbarians and uncivilized. The Mandarin name China, Zhongguo, means 'The center of the world,' which was certainly how they did (and debatably still do) think of themselves.

The sinocentric model operated like so:

China was at the center

Bordering countries like Thailand, Vietnam, Korea etc were tributary states/first or second class barbarians. They offered tribute to China and kept it shielded from the full on barbarians.

From this point of view, it's easy to see why the Chinese saw themselves as the center of the world. They were, they were the center of their world. The Sinocentric model was never challenged until the European powers appeared on the shores of China.