Fairies have been attributed many origins, from natural causes to the darkest element.
The progress of modern science has contributed to the decline of faeries since many phenomenon (storm, bad crop, hunger, disease, ...) that were attributed to them can be explained by natural causes. Other pretend that they simply retreated to their own land, to escape the pollution of our world.
But we must admit that there is still remain something, odd, uncanny and monstrous in our world that resist rationality. Faeries are the creatures of the wild, primitive and untouched realm of fantasy that exists beside each society.