The motto of John Collier's story “The Chaser” might well be “be careful what you wish for.” The “clients” that come to visit the old man think they know exactly what they want. Some want another person dead. The old man has just the thing, a poison (or “life-cleaner,” as he euphemistically calls it) that will do the job with just a small portion—never mind the consequences.
Others, like Alan, want someone to fall in love with them. The old man has just the thing for that, too. It is high-quality stuff, the man asserts, with permanent results, and not just in the physical realm—it is also guaranteed to produce devotion and adoration in the recipient. The man assures Alan that Diana (Alan's beloved, or perhaps target) will care only for him. He will be everything to her. She will hang on his every word and want to know exactly what he is thinking and feeling at all times. She will panic if he is late getting home. She will dote on his every want, never allowing him “to be tired, to sit in a draught, or to neglect” his food. She will be hurt if he is unfaithful, but she will always forgive him. She will be his perfect servant.
Now think about this for a moment. Alan thinks it sounds wonderful. He is filled with joy at the very prospect. But reflect on what it would be like to live with this behavior day in, day out for the rest of one's life. It would become, at best, irritating, and perhaps even intolerable in a very short time. Alan would soon be driven nearly crazy by Diana's attentiveness if he gave her this potion and it actually worked. His life would never be his own again.
Yet Alan doesn't realize the consequences of his desires any more than someone who wants another person dead can see the long-range results of that longing. He is caught up in the moment, positive that he wants Diana's full attention, certain that the life the old man is describing is ideal. Further, readers might wonder how much Alan really loves Diana. After all, he is ready to essentially enslave her without thinking about the ramifications of that one bit. He is ready to change the woman he professes to love rather than accepting her for who she is and loving her anyway (or moving on if their love isn't true).
Indeed, Alan has not yet learned the meaning of the motto “Be careful what you wish for,” and the story ends before we find out what happens to Diana and how Alan will feel afterward. He thinks he is grateful to the old man, but perhaps he will discover that what he wished for is not at all what he needs.