James:

He did.

He realised that he loved you while he was eating breakfast in the Great Hall, admiring how beautiful you looked so this obviously meant he had to yell it across the tables so you would know.

Sirius:

He did.

You two were playing against each other in quidditch and all he could think about was how much he needed you in his life. He flew up on his broom, confessing his love for you as James shouted at him for not paying attention to the game.

Remus:

You did.

You were studying in the library for an upcoming test when you stopped for a moment, admiring the way he was so dedicated to studying. Your eyes lingered on his furrowed brows and then his brown eyes which always seemed to hold kindness within them. The words came out involuntarily as you told Remus that you loved him.

Peter:

He did.

You were in Charms class when he was having difficulty with a spell and you helped him with the incantation and movement, leading to the words falling out of his mouth which surprised both of you. But as awkward as it was, he meant what he said.

Regulus:

You did.

You were sitting in the courtyard with your friends when he you saw him walk into the area with his friends and a wave of emotions hit you as your eyes met. This prompted you to go over and pull him away from his friends. He thought something bad had happened but you quickly reassured him that everything was okay before kissing him and confessing your love for Regulus.

Lily:

You did
You blurted it out in class when she turned around, smiling at you. She smiled even more, saying it back. Sirius overheard and nearly fell off his chair from excitement.

Marlene:

She did

Marlene spelt it out in the snow when you went on a date that was very similar to your first date. You freaked out at first but then hugged her, saying that you loved her too.