How and why did feudalism start ?

[Jonathan]

It didn't. "Feudalism" is a concept to describe medieval society that was invented by nineteenth-century scholars, given half a dozen contradictory definitions over the next century or so, and finally came to be abandoned by medieval historians over the last generation. Its use was first seriously challenged by Elizabeth AR Brown in a seminal article in the American Historical Review titled "The Tyranny of a Construct: Feudalism" back in the 1980s, and thoroughly demolished in the 90s by Susan Reynolds in a book titled "Fiefs and Vassals."

Actual medieval economy, society, and politics was very ad hoc and very localized; conditions that pertained in northern France in the 12th century would not have pertained in England, or Italy, or Poland, nor in 14th century France. In each area there was a different recipe of landholding patterns, lordships and noble kin networks, royal "public" authority, the existence of towns or churches with their own powers and prerogatives, etc. etc. The only common features were a hierarchical social structure with nobles and kings on top and peasants on the bottom, and an economy that was heavily agrarian and reliant on peasant labor, but that was equally true of the Roman Empire, or Greece, or ancient Egypt or China for that matter - it doesn't constitute anything distinctively European or medieval.