On one post, a person suggested that it is a choice when you already had it, but that you lost it. I would agree. I've been trying and choosing to for many years, consistently wanting and expecting love for the first few years, less since then. The last year I haven't been open enough to it from the heart side, not the brain one. The brain one says I will eventaully love her now that we've improved communication & we've both done some changing. My heart, not so much (yet!)
If your wife is saying it, my guess is that she has some blocks/walls that she hasn't broken between you two. For me, my Ws lack of skill (I know that sounds mean...), cultural expections, physical attraction, and mostly her self-esteem have all been walls that I have only partly (or not at all) dealt with.
Some on the boards say that MWD says work on yourself only. My understanding is that you work on yourself, then as things improve (or spontaneously improved by the other partner), topics that bother you are dealt with, not ignored. My IC said that, too, although her focus was on communicating better.
So I'm trying to say that if you think your wife has 'walls' up, then if you understand those walls and can deal with them, or she does, then I think love can develop well. This weekend, I'm praying to deal with a few major bricks in the walls that in my case, have always existed.
Does that make sense? Do you think your wife has got "walls" between you that aren't protective, but just important issues that need to be tackelled?