He's, by nature, not a romantic man. He's a stubborn man, a dense man, too, but not romantic.
But for her, he tries.
It doesn't go smoothly at first. When he tries to brush hair out of her face, his watch gets caught. He gently tries to free himself, but he still manages to pull out a couple of strands. And although she reassures him she's fine, he feels really guilty.
It doesn't go so well the second time, either.
He tries to give her his jacket on a cold night, but he makes one big mistake: he tells her it's cold first. He does this thinking that this is a perfect segway to casually put his jacket on her shoulders without it being random. She agrees, so he's about to shrug the jacket off. Unfortunately, next thing he knows, she's giving him her scarf, and she's peeling off her gloves to give to him.
He doesn't have the heart to reject her offer.
The next time, he is sure that he'll be able to do it properly. He is sure that he'll be romantic.
That fails, too.
This time he goes to hold her hand, which he does without looking because, hey, he's embarrassed to be doing it. But when he feels a hand in his own, he finally thinks he's done it. He did one romantic act.
Too bad it's not her hand that he's holding. It's Ishida's.
All of his other attempts end up the same, and he is forced to accept that he's, by no means, a romantic man.
So why is she telling Tatsuki that he is?
"He…he records my favorite shows so we can watch them together, and he calls me every night to wish me sweet dreams."
Ah. That's why.