[theorique]

It's sort of analogous to a quantum mechanical measurement - you don't know it was unwanted until you try it.

This is why it's better to make an early advance with some plausible deniability, so you can back off if there's no mutual interest (e.g. backing away: "Ha, just kidding ... what, you thought I wanted to date you for real? That's crazy talk ... we're co-workers, and it would never work out, we're so different.")