Love is one of the most powerful emotions that we humans experience. No matter how familiar we are to it, we still cannot predict how or why or when it would occur. But love, as a matter of fact, is NOT blind. It sees but it doesn't mind. No matter how many faults or imperfections we see of the person we love, it won't matter at all because we accept them for who they are. We accept them wholly along with the faults and imperfections, because we, too, aren't perfect at all.
While you are correct that love should always prevail in any situation we are in, truth is, that condition will only be possible in an ideal world. In reality, we humans are also capable of expressing other forms of emotions such as fear, anger, and disgust; a mixture of these emotions is what makes us human. However, if we get overpowered by either one of these negative emotions, that's the time when we go blind. We tend to overlook all the good things, all the good memories, and choose to hate the person we once loved. I think this is what causes couples to break up.