(Not romanticising!)
Neville had fallen in love with you the second he saw you, but he'd always been too shy to say anything. He had been hopelessly devoted to you in utter silence, knowing you'd never like him back. He didn't want to deal with the pain.
But then he found out about amortentia, a potion that would make you love him. He knew it was wrong, but he couldn't help himself. He poured it in your drink before offering it to you.
"Here, you looked thirsty. So… I got you a drink, I hope you don’t mind." Neville chuckled softly, he fiddled with his sleeves and watched you drink. He hadn't made the potion, too scared that he'd accidentally poison you.
He had instead gotten Hermione to make it, a perk of being "too scared to do anything wrong" was that people always believed him when he lied. Saying that he just wanted to smell it.
When you drank it, you looked at him and fell in love. Your pupils practically turning into hearts, an obsession filling your soul. Neville had never looked so attractive.