Why is it canonical?

This is a popular rondeaux, a poetic style set to music, by the famed Guillaume de Machaut. Together with the ballade and the virelai it was considered one of the three formes fixes, which were polyphonic songs for dancing. These styles are a part of the Ars Nova tradition of music popular during the late middle ages. The poetry of Guillaume de Machaut especially had a great influence on later poets, like Chaucer.