[Species X] comes from an alternate dimension, and have come to inhabit medieval Earth. The members of this foreign species have incredibly long lifespans, heightened physical abilities and hardiness, as well as strong penchants for magic. It turns out that through a ‘fusing’ ritual with Species X (viable only with consent of individuals from Species X), humans are able to reap these superior qualities of Species X.
This of course, is beneficial to humans. By fusing, humans are able to gain security, magic, strength, and the consequential military and social prestige following these heightened abilities. However, from the viewpoint of Species X, this fusion has more negatives than positives. Species X tends to look down on consorting with the noisy, squabbling, short-lived humans and their funny antics and materialism. In addition, the ‘fusing’ process depletes Species X’s magic storage, and Species X’s social hierarchy is decided strictly on the amount of magical reserves each individual has.
Some members of [Species X] may choose to perform this ‘fusing’ ritual with humans out of pure curiosity and boredom. Or perhaps some members are human-philes, obsessed with human civilization and psychology. But why would a large percentage of Species X ever consent to work with and ‘fuse’ with humans?
Note: Fusing ritual is not all-or-nothing; individuals can fuse/ de-fuse as they wish. But why would they choose to fuse at all, and continuously so, in the first place?