These two letters deal with the idea of love, or better yet the wrong view of love. If Wormwood could get the man to fall in love through sexual temptation or anything superficial, then the man will fall in love in the wrong way, which will lead him away from faith. The power in this approach is that people will, in time, fall out of love. This lack of commitment will bring hardships. Here is a quote that shows this:
We have done this through the poets and novelists by persuading the humans that a curious, and usually short-lived, experience which they call "being in love" is the only respectable ground for marriage; that marriage can, and ought to, render this excitement permanent; and that a marriage which does not do so is no longer binding.
In letter 19, Screwtape does not want the patient to learn about the love of God. In fact, he is perplexed about this topic. In any case, he directs Wormwood to make the person really fall in love with himself. He reminds Wordwood that people are really selfish. If all else fails, Wormwood should tempt the man to marry a woman that would make his life of faith difficult.
For marriage, though the Enemy's invention, has its uses. There must be several young women in your patient's neighbourhood who would render the Christian life intensely difficult to him if only you could persuade him to marry one of them.