Guild Girl is an empathetic individual, showing concern over the villages pleading for help against goblins and rookie adventurers confidently attempting goblin quests, as shown with Warrior's party. Understanding how dangerous goblins really are, Guild Girl muses to herself how goblin attacks are on the rise and villagers are desperate for help, but lack the funds to hire skilled adventurers. Knowing how often rookie adventurers get killed or worse attempting goblin quests, Guild Girl holds much contempt for most silver-ranked adventurers other than Goblin Slayer, as he is willing to take any goblin job, while other adventurers refuse them due to the low reward, lack of fame or disinterest.
She has developed a dislike for boisterous individuals, due to dealing with them on a daily basis, and has developed a type for silent and stoic men. As a result, Guild Girl develops feelings towards Goblin Slayer (being the only adventurer taking goblin quests and being the least boisterous among adventurers).
Source: Goblin Slayer Wiki