(CNN) -- Miley Cyrus clearly knows the way to Liam Hemsworth's heart: The actor tells Details magazine that he's smitten.

The couple got together on the set of their movie "The Last Song," and Hemsworth, 22, tells the magazine that they've been together for two-and-a-half years.

"What happened happened," the "Hunger Games" star says. "She makes me really happy. When you start, you want to be professional, but when you're filming those scenes with someone and pretending to love them, you're not human if you don't feel something."

Lately, Hemsworth has been feeling protective of Cyrus - her 19th birthday, he points out, turned into an excuse for someone to share an offhand comment via video that quickly went viral.

In November, someone captured the former Disney star on tape referring to herself as a "stoner," and Hemsworth defends what her rep has explained was a sarcastic remark.

"She's in a room full of her best friends, and you have one person who comes in there and videos it," Hemsworth says. "The poor girl can't have one night where she can feel safe in her own world. It's ridiculous."