One of the oddest parts of building a relationship is the creation of a set of remembrances; souvenirs from the trip that romance takes you on. Two people can create a lot of these symbols over the arc of a love affair. Poetic Memory (the memory that sets apart love from friendship, according to Milan Kundera in the "Unbearable Lightness of Being") is littered with these tokens, they can be knickknacks, events, ticket stubs, songs, or even shared ideas. Years after a love has stumbled these things can bring a participant back, longing for the Ever-Greener-Grass of Way Back Then.
These objects form a border around two people, and other people can't get in. This can be one of the problems young relationships have; sometimes it feels as if a new romance can never build up the library of objects equal to what previous relationships have established. Falling head-over-heels might be a protective mechanism that insulates a romance until it has built up an object-vocabulary of its own.
For anyone who takes the time to look, here is an partial list of "Wistful Associations" that I put together and keep in my electronic journal. For the Digest, I've dissociated the concepts from the people I've shared them with: I'm a bit of an emotional exhibitionist, but can't be sure that someone I've loved doesn't think of these as a kind of sacred space.
In (roughly) chronological order: