I think she intrigued him. She was the only (sane/non-evil) person he knew who didn't like/trust Superman. And she liked him for Clark.
Both were fairly new experiences for him.
Having someone with a good heart who didn't trust Superman immediately and implicitly helped keep him honest. It's a danger, when so many people believe in you so strongly, to a degree that you're not sure you can live up to. A danger that you'll let it go to your head and take things too far. And a relief, to have someone who isn't part of the fawning crowds when you don't really enjoy the spotlight (and, deep down, still aren't fully comfortable with it), someone who doesn't believe that just because you're a superhero, you're perfect.
On the Clark side of things... Here's an attractive woman who's interested in the nice, shy, quiet guy. We don't know Clark's past fully, but I get the sense that it's not something he's used to. There was Lana, but she was wrapped up in her own world. And maybe there was someone in college, but it can't have been anyone too major because he left to go travel the globe, alone. And, too, he had his secret to hide and no way to let it out, which probably made him more shy, private, and reclusive.
So... as Lara said, I don't think he was even "in love" with her, that he ever "fell" for her. But he liked her, and he found her intriguing, unique, and flattering.
Also, he didn't have much choice about it. The writers wanted some tension...
