Spot the Signs. The most obvious sign that you'll receive this reaction is if you haven't had a healthy relationship with your manager in the past. Maybe you continually challenged his ideas, and he took that as a sign of disrespect. Or, maybe she always made unreasonable demands, and you were never able to see eye_to_eye on your to_do list. Or perhaps his moods were completely unpredictable. In any case, if your relationship has been strained, you may receive a less_than_ideal response to your two weeks' notice.
You want your intent to be as direct as possible. You are leaving, plain and simple. There's no need to mention where you're going. You can discuss that with your boss (though you're not obligated to do so) and your colleagues if you want, but there's little purpose for including that in your resignation letter. That's especially true if you're moving over to a competitor__you don't want to create ill will in this letter or cause any drama.