There is a trope in romance that when you meet the person you are supposed to be with, you can talk about anything. You can apparently inverse that too: if you talk about anything that pops into your mind, you can tell if you're supposed to be with the person by judging their reaction.

There’s a real tension in love — at the beginning of love, particularly — between the desire to be honest about who one is and the desire to win the affection of another person. Of course, ideally, we can both be honest and loved for being honest. That’s the dream.