Loving someone is different from being in love. It can get you through when the infatuation/passion is lacking. I really don't think many people manage to be dewy-eyed romantics 100% of the time. For myself... this is kind of sad to say, but as long as I have depression, I'll have periods when I don't really feel "in love." The two things are just so contradictory, and when you're that down, you don't like anything much. But if the person I care about ever needed me during those periods, I would still do everything I could to be there for him, and I hope he knows that. Love is constant even when nothing else is.
That said, I do like to think that people can be together for many years and still have those really sweet moments of hand-holding or appreciating each other, as well as a genuine sexual attraction. If I thought that were impossible then I would just stay single.