If the holder becomes infatuated, the Pokémon that the holder is in love with becomes infatuated as well.
Outside of battle
From Generation VI onward, if held by a Pokémon in a Day Care, Nursery, or during a picnic, any Eggs produced by that Pokémon will inherit 5 of their individual values (IVs) at random from either of their parents, instead of the 3 they normally would. Each IV is independently inherited from a random parent.
An item to be held by a Pokémon. Should the holder of this bright-red thread become infatuated, the Pokémon it is infatuated with will be similarly afflicted.