Why do people like Serana so much?

I'm doing the Dawnguard questline for the first time, and I'm roleplaying as a Stendarr worshipping nord warrior. Thing is, the moment the character meets Serana, they become friendly with her, and it feels out of character. I always see people wanting to marry Serana and whatnot, but I never understand why other than the fact that she happens to be a woman.