Luna has the ability to contact those who are dead, but haven't chosen to move on from their death. For ponies that try to make others lives feel great she gives them a choice for if they want to stay and help, or leave and see what is beyond them. Eventually leading to extreme amounts of stress creating the mare know as Nightmare Moon.

Edit:
For those reading/starting to read: I don't update often, because I'm studying to become a software engineer. I don't have much time to write, or even to have "fun" at all. I'd appreciate your patience, and thank you. :D