Lily is Jarrod's guardian angel.
Because we need some reason for Lily to want Jarrod. Jarrod acts like a complete jerk all the way through, and Lily can do better.
Towards the end of the film, it begins to become clear that the kind of unconditional love Lily gives Jarrod is exactly what he needs, and perhaps the only thing that can save him. But this doesn't explain why Lily likes him. The average human finds unconditional love for another person hard to imagine, let alone pull off
Unless she's his guardian angel, and what's good for Jarrod is her only
concern. She's upset when he rejects or ignores her because she can't complete her task if she doesn't have access to him; but her own personal feelings aren't involved. She simply has a duty to watch over Jarrod.