Death's job is simple. When somepony dies, it's up to him to make sure they pass on smoothly. Equestria isn't that large, and so his job is normally simple. A few here and there aren't ready to move on, and he helps them as much as he can. For him, life, or whatever it is you want to call it, is simple. Until Princess Luna, the only living being who can freely acces his little office, shows up with a request.

After all, Twilight is an alicorn now, and certain pieces of paper need to be filled out.