Statement used to express regret and or fear about the current situation of an amorous or once amorous relationship. It generally means that the person who is expressing himself/herself has a deep emotional bond with their current mate, but that bond is possibly/probably not strong enough to merit a lifetime, or long-term commitment. Usually, there is either some dissatisfaction, disappointment, or confusion on the part of the person using this statement.
Woman: Why can't we be together?
Man: I love you, but I'm not in love with you.
Woman: What does that mean?
Man: It means that I need something in my life that being with you is just not able to provide. If you need anything, I'm there, but we aren't working.