Problems encountered while writing.

I just started writing a passage where my friend has just seen the girl of his dreams, but it's difficult to say why he likes her.

In The Club Dumas, Balkan also sees an attractive girl from across a crowded room, who eventually becomes embroiled with his friend Corso, the main character. The rapport between the girl and Corso is established in three parts. First, they are in the café discussing the relationship between Rochefort and d'Artagnan in The Three Musketeers and:

She was wearing a blue duffel coat and carried a pile of books under her arm. Her chestnut hair was cut short, like a boy's. Now she sat at a slight distance, not quite part of the group. There are always a few young people at our table, literature students that I invite for a coffee. But this girl had never attended before. It was impossible to forget her eyes.

Then, they meet on the train and even then, it's not clear that Corso likes her:

"I like trains," she said.
"Me too."
The girl was still facing the window, touching it with the fingertips of one hand. "Imagine," she said. She was smiling nostalgically, obviously remembering something. "Leaving Paris in the evening to wake up on the lagoon in Venice, en route to Istanbul..."
Corso made a face. How old could she be? Eighteen, twenty at most.
"Playing poker," he suggested, "between Calais and Brindisi."
She looked at him more attentively.
"Not bad." She thought a moment. "How about a champagne breakfast between Vienna and Nice?"
"Interesting. Like spying on Basil Zaharoff."
"Or getting drunk with Nijinsky."
"Stealing Coco Chanel's pearls."
"Flirting with Paul Morand... Or Mr. Barnabooth."
They both laughed. [...]
"Trains aren't like that anymore," he said.
"I know."

Then they bump into each other again at the hotel in Sintra:

"Won't you sit down?"
He did so, vaguely anxious. The girl shut her book and regarded him curiously. "You don't look like a tourist," she said.
"I'm not."
"Are you working?"
"Yes."
"Any job in Sintra must be interesting."
That's all I need, thought Corso, adjusting his glasses. Being interrogated after everything I've been through, even if it's by an extremely young, beautiful girl. Maybe that was the problem.

Admittedly it's too devoid of emotion, but it's a great example of how to introduce a love-interest where you've got nothing to go on but her looks.