It's one of those things that's really difficult to narrow down to solid factual qualities when someone demands specific justifications for liking it (MLP, for those who like it, is similar. I can understand the comparison with love too).

I'd say it's a feeling of charm and excitement and that there's no single cause for it or even a small number of causes, but a wide, almost organic range of influences. Obvious ones are music, relate-able characters and story, but also a lot of little things that may even be subconscious. A "chemistry" or "soul" if you will.