And sure, this could be an email (all of the same principles would apply), but a letter makes things official in every sense. It also gives you a prop to take along to the face_to_face meeting with your boss, when you let him or her know that you're leaving.
Spot the Signs. You may receive this reaction if you're a high_ranking executive or director. These positions can be difficult to replace quickly, so it may be worth it to your company to tempt you to stay rather than letting you go without a fight. It can also happen if you have extremely specialized knowledge that would make it challenging to replace you. On the other hand, if you're in an entry_ or mid_level position, and someone could be hired into your position without much trouble, it's less likely your boss will put up a fight__and a counteroffer.