Steve
You were reading a book. He was looking at your messy hair, eyes that were scanning pages and biting lip. Steve suddenly realize I love her.
Tony
You were making dinner. He tried your food and he was amazed. Everything was amazing. Tony realized that he loves your cooking and especially you.
Bucky
He realized he loved you when you comfort him after a nightmare. You told him he was loved and that all of that is in the past. He was in love with you from that moment.
Pietro
He realized he was in love with you when you first sassed him. He was a master of a sass and you sassed him. He was in love immediately.
Bruce
Bruce realized when you calmed him down. He was minutes away from hulking out and you calmed him. He was whipped.
Thor
He realized he loved you when you pick up his hammer. That mean you are worthy. He was amazed.
Clint
You were shooting for the first time from his bow. The arrow went straight to center. Clint realized Damn... I love her.
Loki
Odin was yelling at him for being useful. You started defending Loki saying he is strong and brave. Loki was immediately in love.