Why do willow trees propagate so easily?
The technical explanation is that willows (Salix) have preformed or latent root initials that will elongate into an established root system when a part of the parent plant is removed.
If you were to take cut willow branches and make a kind of tea by soaking them in water, that water could be used as a sort of natural rooting hormone to help root other types of plants. This indicates that willows naturally contain a high level of the hormone that contributes to root formation.