Even if you're not close with your boss, chances are you have a working, first_name_basis relationship. Maintaining that in your official letter is okay. You don't need to keep a Victorian level of formality here__just make sure it's a professional tone.
Spot the Signs. Again, you may receive this response if you have specialized knowledge that would make it difficult or time_consuming to hire your replacement__or if it would be helpful for you to be actively involved in that search.