Why was military affairs in the Middle Ages the main occupation of the feudal lords?

The structure of feudal society assumed the supremacy of the feudal lord. He was the supreme person who disposed of the destinies of his subjects at his own discretion. Although this way was not as tough as the slave-owning, the fate of a simple peasant was difficult: to cultivate his land allotment and give almost all of his birth to the feudal lord (seigneur). Naturally, the feudal lords wanted to expand their possessions, so clashes often arose between them. Good military training guaranteed a positive outcome of the battle and the expansion of fiefdoms.