This is a powerful line for two reasons, the first being that it's a reminder of what these two seemingly polar opposite characters have in common. But also, it's the thing we've been waiting six seasons for them to realize. This blindness is what held Hannah back in her personal essay writing (among other things), and it's what drove Marnie to make bull-in-a-china-shop decisions in every area of her life. It's somewhat of a surprise to hear them say it so explicitly, but it's also a long time coming.