She remembers the heady rush of chips, but coupled with the assurance of statistics. She remembers the sensation of 'beginner's luck,' and Booth saying he understood, but she hadn't really understood herself. It had been a momentary lapse—her time at the tables had only reassured her that she would never understand gambling. What could entice someone to take odds against their favor?
But she understands now. She understands now the desperation, the clutching to irrationality, and belief in luck and chance over fact and reason.
Because she's gambling that what they have will be enough to make him stay.