It's common for the passion, the romance, the interest, and even the connection of a relationship to die down over time. By design, the excitement of someone new diminishes.
Sometimes the cause is a change in lifestyle: kids, a more demanding job (or commute), financial stress, or a loss of some kind. Sometimes it's the lack of anything new. Perhaps you are experiencing subtle or more obvious physiological changes.
Maybe over the course of events, you realize this isn't the person you thought. Perhaps you have grown, leaving the other behind in some way.
Sometimes you're not even sure where, when, or why the warmth and excitement was lost.
There is the love you fall for...and that falls away, and then there is the love that you build and that you stand for. I assist couples in making the transition from one to the other.
Forming relationships is an active and creative process that calls on our ability to participate in our own evolution and that of our relationships.