This is really interesting. I think I could certainly choose to love someone less or more, but I'm not sure I could turn it off completely. I suspect that like for my ex I would always have fondness waiting to be reawakened if that makes sense.
I can choose every day though whether to see my husband in a positive or negative light. I can tell myself internal stories about his motivations that tear holes in my adoration of him, or I can watch him, assume he is doing the best that he can always and feel myself full of admiration and longing for him. I think we have a choice and I think people choose sometimes not to make the effort to keep loving, because its not always easy over the longterm even if you are totally compatible.
I watch my Mum recently in her second marriage, believing that love will and should just happen and she shouldn't have to make an effort. Kind of like, love me the way I want you to, and until you do that I have nothing to give to you. I watched my parents marriage struggle along these lines, neither of them willing to put any effort in until all their needs were met by the other person. I think sometimes we have to just decide to love someone through times that they may not be capable of loving back the way we would like, but then I'm married to a man who decides to love me in this way all the time too. I suspect my ideas may fall apart if he were different.