Turning in your two weeks notice can be terrifying__because no matter how well you think you know your supervisor, you can never be 100% sure of his or her reaction. Will he be furious and insist on walking you out the door immediately? Will she beg you to stay, enticing you with an enormous raise? Or will he be completely civil and wish you the best of luck?
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.