I think it's definitely possible to "fall out" of love when the relationship isn't being maintained properly. At some point the negatives and stresses and resentments overwhelm the positive features and make it hard for me to see them even if they're still there.
At the same time I don't think the end to a romantic relationship has to be accompanied by the loss of love, and I've found that I'm much happier when it isn't. Being able to still feel love, even if no longer in the romance, helps me to believe/remember that the relationship was real and valuable even though it ultimately ended. And looking back at relationships in a positive fashion, rather than a negative or cynical one, helps me to have faith or trust in current loves.
Of course, feeling love in ending or shortly after a romance is hard. I have positive feelings now that were swamped by the negatives for years; it's only recently, when I managed to talk about the relationships with the former partners, realized the significance of miscommunications, and could erase some of the negative feelings, that I've been able to feel that the love I had felt is still present. But at least I finally got to this point; it feels like a wonderful gift.