In letter 25, Screwtape hits upon an aspect of humanity: the desire for change or novelty. Wormwood must encourage this desire for change in his patient, because it is this desire for novelty that causes heresies, marital unfaithfulness, breaking up of friendships, and so much more. To put it another way, this desire for change cuts the legs of faithfulness, so that it cannot stand. Here are Screwtape's own words:

The horror of the Same Old Thing is one of the most valuable passions we have produced in the human heart—an endless source of heresies in religion, folly in counsel, infidelity in marriage, and inconstancy in friendship. The humans live in time, and experience reality successively.

The irony here is that Screwtape also acknowledges that the enemy (God) has created rhythms. So, Wormwood must not allow these rhythms to be experienced, which only happens by steadfastness.

In letter 26, Screwtape urges Wormwood to do two things. First, he must sow seeds of discontent while the couple is still courting each other. While there is erotic attraction, the couple will sacrifice for each other, but the time will come when this feeling will pass. If seeds of selfishness can be sown now, they will cause many problems later.  The subtle way this can be done is by allowing both people to believe that they are being selfless, when in fact they are acting for their own gain. Bitterness will be the result.