Why are the Middle Ages sometimes referred to as the Age of Chivalry?

The whole idea of Chivalry arose during the High Middle Ages. It was informed by people such as Eleanor of Aquitaine and a host of European bards, who used the idea of the good warrior, derived partly from the Arabs and partly from the crusades, as a basis for songs and stories.

With the end of the Middle Ages, the knights passed out of usefulness, and the age of Chivalry ended with Don Quixote, which, while very funny, mocked the idea of the romantic knight.