Because she is –
Beautiful? But she's not, she's not beautiful, not really. Her hair is not sleek and smooth and shiny and soft, but untamed and wild. It frizzes up in the rain and even on dry days it's a tangled mane. Her lips are not ruby-red and her eyes are not emerald green. Her lips are an ordinary dull shade of reddish-pink that are sometimes chapped and her eyes are brown. Ordinary brown, like her hair. Nothing spectacular about any of these features, and yet – and yet when she smiles, or when she's frowning over a textbook, or her forehead screwed up in concentration as she writes an essay, or in those rare moments when you catch her daydreaming, she is. Beautiful.
Smart? And yet you've never been one to be impressed by the ability to memorise passages out of books, or read ahead and fool everyone into thinking that you're oh-so-smart just because you've read a chapter they haven't, and to be quite honest her intelligence and her incessant need to display her intelligence annoys you. And yet you forgive her for it, because it's her and somehow you can't manage to be annoyed with her for very long.
Politically aware? No, good grief, that can't be it. Her social conscience and opinions irritate you even more than her cleverness. You don't agree with her stand against house elves working, because you don't think she understands it fully. She has good intentions, but she's looking at it from a Muggle perspective, and she still doesn't understand that things are different in their world. She doesn't understand that they don't want to be free, and she has ridiculous opinions on other things, too, like Quidditch being pointless, which made you want to slap her across the face and scream at her, but instead, to silence her, you kissed her.
Not afraid? Because she wasn't afraid to kiss you back, or to tell Ron and Harry without the slightest hesitation that you were going out. Because she isn't afraid to love you, and to tell you that she loves you, and she isn't afraid to let you see what she's like beneath the messy hair and the know-it-all attitude, or of seeing the you beneath the surface. Because she has that Gryffindor bravery that you know you should have but debate sometimes whether you really do or not.
Because she is –
Maybe that's it.