There’s a German word for it, of course: Sehnsucht, which translates as “addictive yearning.” This is, I think, what these sites evoke: the feeling of being addicted to longing for something; specifically being addicted to the feeling that something is missing or incomplete. The point is not the thing that is being longed for, but the feeling of longing for the thing. And that feeling is necessarily ambivalent, combining both positive and negative emotions.