IMAGINE GOING TO A SCHOOL REUNION WITH GRYFFINDORS AFTER THREE YEARS YOU GRADUATED HOGWARTS AND SIRIUS BEING IMPRESSED ABOUT HOW MUCH YOU'VE CHANGED.
Sirius remembered you being that person in the class who was always picked on, who didn't enjoy Hogwarts as much as the most of the other students did. Sirius hadn't ever picked on you and towards the end of the time in Hogwarts, he even hung out with you because Lily was your friend, even though you weren't his friend, you had had feelings for him, but never said them aloud. You usually were there, doing something and mostly ignoring everything that happened around you. You just sat there and always looked serious or pissed.
But you had changed. You smiled more, talked more and overall, you were a pleasant person. Only your looks had told Sirius it was you. He smiled at you at the restaurant as he listened to you rambling about your new work and the life you lived, how you felt happy. He hadn't expected you to turn out like that, he hadn't expected that you would have that kind of life. But he was happy for you - you seemed like you lived a happy life, you had gotten friends, you had gotten more social and you weren't that person who was always quiet. You laughed with others as someone told a joke, you didn't go into the shell anymore, pretending you didn't exist. Your smile felt like it lit up the whole room and Sirius was impressed about your change, he would have never expected you to turn out like that. His schoolboy nature lifted its head a bit as he made a flirty joke at you, and you blushed, remembering your old feelings - and Sirius felt something too upon seeing your blush. You had changed, and now Sirius saw you differently - and maybe on that day, he developed some feelings towards you himself too.