This is always the cutest joke, I love that Setsuna doesn’t go for the “he loves you he would never look at anyone else” line that’s often used in situations, she goes for “you have the very quality you just referred to“ which is so much more helpful to Usagi, who I think can’t conceptualize people loving her enough not to leave as an idea unto itself.

It’s such an understated way to imply that since Mamoru has Usagi, he doesn’t need anyone else. Right before this, Usagi calls Setsuna a mom, but I think this puts her more as an older sister, someone who has a little more wisdom, but is still close to how you feel in the moment, and I love Setsuna getting that role, she’s still set apart from the others, but she can connect to them on a level we don’t really get to see elsewhere.