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.
A resignation letter can also help your boss and your company start the transition process, get a replacement for you approved, and start planning internally, etc. The paper trail is beneficial all around. So what should go into your letter? Let's look at some examples of what to do (and perhaps more importantly, what not to do).