1. Spending Too Much Time Together
When you spend too much time together, alienating everyone else you know, you don't just become THAT couple, but you run the risk of losing the other people in your life. 

How to break it: Set aside at least one day a week in which you see your friends and/or family. Not to be pessimistic, but if your relationship doesn't work out, you're going to regret blowing off your friends — they might not even be around anymore when you need them.