...What does this mean?...
Honestly, it could mean a lot of things. How badly do you want to know the answer? Getting to the WHY of this kind of behavior is an exploration that sometimes takes a bit of time. The things that can contribute to this behavior are many: past relationship experiences, the unique conditions of that encounter, family history, the experience of your current relationship, your understanding and experience of "love", etc.
The attraction power of that one-night stand has probably morphed into something beyond reality over the past year. That doesn't diminish how strongly it feels to you, but the imagined bond you have with that person is much greater than the actual one you have (or could have had).
There was something about the longing connection you felt (or remember feeling, even if it wasn't actually that powerful at the time) that you still desire. But people who go after a love of longing often miss out on love of devotion. By its very definition, a one-night-stand involves no devotion.
Want a good book to read? Scary Close by Donald Miller.