@Meroko26 as an angel who had a job for eternity, maybe she knew that IF Oji-san died there's high chance they can't be together. there's many opinion about afterlife like fast cycle reincarnation, late cyle reincarnation (the soul putted on endless river of life etc.), live in heaven where the angel wouldn't know which layer (floor/stage in heaven) Oji-san will be putted or maybe can't even enter it.
Anyway, angel-chan think that if her job and wish already fulfilled then she can't be together with Oji-san anymore.