Can you tell us more about your situation? How long have you been with this person? Are you married? How long has it been since you felt "in-love," and were there any major events/circumstances that have changed the nature of your relationship?
(I hope I am not being too nosy, but it's just hard to answer your question without a little more specific information.)
Personally, though, I do think the "in-love" part comes and goes, and that it really requires work and conscious effort to keep that part alive. Romance is hard work, and keeping it going for years upon years is not something "automatic" (maybe I'm a cynic, but I know very few people who succeed in relationships by taking each other for granted).
I think Sultana is right in that if falling out of love is the only reason for leaving a relationship, you might find yourself in the same place with the next one, and the next, etc... some people never get out of those patterns, and they are never happy. Now, if you sincerely feel like you are "settling" and that you are compromising your own happiness only to remain in a committed relationship, and not because you are truly committed to the other *person* (not just the relationship), then you have to ask yourself what you really want and if something/body else would make you happier. But if you do really love that person, then maybe it's just a stage (as Shesus said) and there will be more in-loveness in the next stage.