I'm currently plotting a fantasy story inspired by celtic fairy mythology. In celtic folklore, there are many fairy lovers, that is, romantic or sexual relationships between humans and fairies. Usually these relationships are forced. Either a human male forces a fairy female to be his wife, for example by taking away and hiding her swan feathers or seal skin. Or a fairy male forces himself on a human girl he encounters on his grounds or lures her with promises of love only to kill her.
In my story, I would like a female fairy to willingly enter into a romantic relationship with a human male (i.e. "marry" him), without guile or force on either part. But the problem I have is motivating why a fairy would want to live with a human partner. So:
Why would a fairy want to marry a human?
I am intimately familiar with celtic folklore and the fairies there have no inherent interest in human-fairy romantic relationships. My question therefore does not stem from a lack of research, and you don't need to be familiar with celtic folklore to answer my question. Let me explain: Fairies in celtic mythology are not human. They either exemplify abstract moral principles (and reward politeness, helpfulness, diligence, and so on) or have the alien mind of forces of nature, that interact with humans in the same way an animal or storm would. The motivation I seek must be in tune with the fairy's non-human character. I can therefore rephrase the question a bit:
Why would the personification of a moral principle or the personification of an aspect of nature enter into a romantic relationship (or something that appears as such) with a human?
I understand that questions about actions of individual characters, character building, and elements of plot are off topic on this site. My question, though, is not concerned with the actions of an individual character or the building of the personality of an individual character, but rather the general motivations of a species in my world. Your answer, therefore, shouldn't apply to one random individual fairy (and, for example, suggest that unlike other fairies this specific fairy might find fairies unattractive and humans attractive), but must explain either a general human-mating behavior pattern in fairies or point out certain characteristics in fairies that under the right circumstances lead a fairy to wanting to marry a human.