I've been reading John Owen's treatise on the mortification of sin. Owen is incredibly comprehensive and Christ-centered. At one point he pondered why God does not immediately remove some besetting sin as soon as we implore Him. He answered with the following points.
First, our struggle with some particular sin is usually caused by a general failure to "watch" our relationship with God. A man's struggle with sexual lust may not be caused by television, movies or a scantily clad co-worker, but by a general neglect of prayer and the regular hearing of the word of God.
Second, the propensity to neglect our relationship with God should be more grievous to us than the frustration of any particular lust.
Third, quite to the contrary, we are often unaffected by the neglect of our relationship to God, while we come under terrible conviction for succumbing to particular lusts.
Finally, Owen suggested that for this reason God does not remove the besetting sin from us. Instead, he uses the conviction of this particular sin to awaken us to our poor general condition. This sin actually becomes the occasion for a renewal (even if brief) of our pursuit of that relationship, as we come to God, confessing our sin and weakness, praising His mercy and grace, and clinging to the Crucified as our only hope.
Indeed, God is working all things together for the good of those who love Him, who are the called according to His purpose. Once again I am reminded that I am a studied sinner and Christ is a steadfast Savior.