If you compare Europe and China in a historical context, they both share a few commonalities. Different states in Europe retain different cultures, languages, currency, nationalism, etc. The same can be said about all of the Chinese states during the times of a fragmented China. However, it seems that while Europe has never really been under the control of one nation before (outside of brief military conquest), China has existed under one flag several times throughout history. So my question is, why is China so much easier to unite into one nation than Europe?
If I recall: the Mandate of Heaven under which the emperor reigns is the natural order, and no one leader can call himself the emperor without controlling the entire country. So after the first empire in 200ad ish, regional actors would seek to unify at all costs, knowing their neighbours would be doing the same against them. Therefore periods of empire, and periods of warfare. Always one or the other.