If the Chinese were ahead of Europe technologically speaking by centuries?

Question

What would be some of the causes for the (relative) decline compared to the Europeans.

Answer 1

The Chinese may have very well been technologically ahead of Europeans for millenia, however, because their lifestyle favored de-centralized agricultural society, rather than intensive/compact city dwelling, eventually their efforts petered out...

Europe, being much smaller, relied on powerful cities alongside major rivers, which combusted into higher technological levels of thinking, in order to overcome the problems of mass sanitation, building/architecture, transportation as well as healthy-living (To prevent plagues)!

Answer 2

China developed an insular society. They felt they didn't need contact with outsiders and felt all outsiders and their culture were barbaric.

In the 1400s China did send fleets out as far as Africa. However, with regime change, there were no more fleets.

Most of China's rulers wanted simply to stay in powers and one way to do so was to not let new ideas and  technology in.

Europe had a number of smaller states that were constantly feuding. They traded, improved their technology and were (mostly) open to new ideas. Had one person united all of Europe then what happened in China could have been repeated.