Need to justify military power in context of Christianity!
So Just War eg Pope Urban calling for crusade to recover Jerusalem.
Expected to show same qualities of self discipline, sacrifice as other
pious people, but other things crept in eg largesse, courtesy and
eventually courtly love.  Anglo-saxon warfare usually involved killing
your enemies, in France became more acceptable to accept surrender
(less expensive in terms of lives, yet also encouraged constant
fighting).
Love was seen to ennoble the fighting man, and found
echoes in the figure of Mary, a popular choice for knights.  At least,
felt to be valuable for young knights off on expeditions to the middle
east or baltics.
Chivalry began to fall apart as knights lost their military power eg against spears of Scots or pikemen, later artillery. But then began to be idealized as figures of Victorian gentlemanly values (and also as bastions against tyranny eg Ivanhoe in Walter Scott).
BBC Radio 4, In Our Time.