Katsuki wanted to be nice to you.
Like in all those manga he read. He wanted to compliment you, give you nice gifts, and kiss you. Kissing you all the time. On your forehead first thing in the morning instead of greeting you, on your nose when he met you randomly in the hallway, on your neck and shoulders when you watched movies, kissing the back of your hand when you were nervous and biting your lips in the hallways during recess.
But he didn't. His max was to text good morning and whisper a couple compliments during foreplay at night. And, oh shit, he hated himself for it. He knew how much you loved all that romantic nonsense and how much love you had for watching other people in love.
But he couldn't.
He was dying on the inside from embarrassment and anxiety when he did that. Yes, you would never judge him for it, he knew, but it didn't make the situation any easier. But why not give it a try, right? For your sake.
You watched a movie at his house while his parents were away on a business trip and relaxed after the school week. He sighed, trying to calm himself down before tucking his face into your hair at the back of your head and gently nuzzling your neck, hoping you wouldn't think he was being silly. His husky embarrassed voice reached your ears and you heard quiet compliments.
"...You're very beautiful. And I love the way you laugh, like you did at lunch today... and... your uniform looks even better on you..."