Well, I feel there are several reasons for going dark, and some of them are FRT (First Resort Tactics).
1) Because you cannot control yourself when talking to your SO. Maybe you get in arguments, maybe you beg and plead. So, you limit the contact and find peace within yourself.
2) Because they cannot control themselves. Why listen to someone berate you or try and control you?
3) To be mysterious. Make them wonder what you're up to. Works best with grey rather than dark. (Sorry I wasn't home last night to get your call. Huh? Oh, I was just out with some friends. Oh, I was out at the movies.) I actually did the movie thing - I'd go to catch a flick, come home to find a message. Then get the grilling - where were you? Who were you with?
4) You're done with them and would rather listen to Barry Manilow do Metallica then talk to them ever again.
5) You forgot to pay the phone bill.
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