He Looks For Opportunities To Hang Out With Her Alone ... Without You
If there's always something that the two of them have to do together alone, keep a watchful eye on their behavior. They might try to justify it by saying that this is their "friend thing", or that they've always done this together and you can join them next time. Maybe it's true, but don't let them blind side you with a dramatic turn in your relationship where you find yourself alone.
[Photo via @summerbreak]