Perhaps it is the side effect of humanity's nature; our natural desire to rebel against all forms of authority, even if it were ourselves that placed these restrictions upon ourselves in the form of such a resolution. Why do we bother trying, we should stop and ask ourselves this very question often. More often than not, we become so accustomed to going about our daily lives without bothering to try and understand the meaning behind most of what we do. We are merely content to go about it, or give up after trying a few times.
We simply cannot look to ourselves to change, or hope it brings about lasting change. If we manage to change by willpower, when we inevitably die, all that change amounted to nothing. Something has to make us change. An external factor that tells the heart, "that which you desire is wrong." And only then can such a change begin to happen within ourselves because it will not simply be ourselves but would undoubtedly affect others as well. Our New Year's resolutions fail to change us because these aren't designed to change us. And that's why everyone breaks their New Year's resolutions, because they're designed to fail from the very beginning.