Maybe because you like the model, or concept art or the story that surrounds one, or maybe you love mysterious vibe about one?
I personally am very, very interested in Deep Sea Dragon, I like the mystery and the creatures in the Ocean can actually be very cool, also Largos' lore interests me and in PS there was always something ominous about them.
I don't want the Ocean itself expansion, but rather I'd love a spooky, ominous domain of the Water Dragon full of creepy, lovecraftian buildings surrounded by a mystical mist that would rise from the Ocean itself.
But if Tyria herself is an Elder Dragon, then I'd be all about it.
The oldest and strongest emotion of mankind is fear, and the oldest and strongest kind of fear is fear of the unknown.
H. P. Lovecraft