A deep question for couples is one that reveals how a partner makes sense of their world, not just a fact, memory, or feeling they haven't shared before.
The 36 Questions study is a 1997 psychology experiment that used escalating self-disclosure questions to generate feelings of closeness between strangers in a lab, not a formula proven to make two people fall in love.
The ideal question frequency for couples is roughly once a day or every few days, asked in small doses, rather than occasional long sessions that try to cover everything at once.