The old man guarantees that the love potion will produce all the effects of idealized romantic love. Do you find the sort of life he is describing for Alan and Diana appealing??
Yes and no. The trouble with idealized romantic love is that it doesn't last. That is precisely what the story is all about. That sort of love, including the physical part of it, is very appealing for a while. This might be called the honeymoon phase of a relationship. The two lovers can't get enough of each other. John Collier describes it perfectly through the lips of the old man.
"She will want to know all you do," said the old man. "All that has happened to you during the day. Every word of it. She will want to know what you are thinking about, why you smile suddenly, why you are looking sad."
"That is love!" cried Alan.
"Yes," said the old man. "How carefully she will look after you! She will never allow you to be tired, to sit in a draught, to neglect your food. If you are an hour late, she will be terrified. She will think you are killed, or that some siren has caught you."
Another short story in which the author deals with the same theme is Roald Dahl’s “Lamb to the Slaughter.” Patrick Maloney is tired of the claustrophobic relationship with his wife. They have run out of things to talk about, and their sex life is probably quite dull. Even nonexistent now that she is expecting a baby. He tells her he wants a divorce. Mary must have become disillusioned her domestic routine, her dependence, and her confinement to their little house. When she suddenly kills her husband with a frozen leg of lamb, it shows her own unconscious evolution from romantic love to recognition of the grim reality of marriage.
Romantic love is great while it lasts! In fact, it usually takes a couple a long time to realize that it expired some time ago.
There lives within the very flame of love
A kind of wick or snuff that will abate it