It's typically an understood rule for all magic. For there to be results, there must be a possibility for it to happen without magic. Example, I would not do a money or job working when I never apply to jobs, call businesses, and I sit around the house all day doing nothing.
Songs of Solomon 4:11-14 gives a decent example of a honey jar, however, do not use milk for a longer work because it will spoil. Being a sympathetic practice, spoiled milk wouldn't be a good representation for love.