A Professional Greeting. Your letter should be addressed to your boss. There's a good chance that this letter will merely end up in a file in HR, but it starts with your boss. Assuming this is someone with whom you work reasonably close with on a daily basis, it doesn't have to be super stiff and formal. Specific and professional is the right tone here.
However, it could also be the reaction if your company is approaching or is currently in a busy season and needs the additional staff, or it's experiencing unusual circumstances, like a hiring freeze, which would prevent it from replacing you entirely.