She likes the idea of him. She was always the little sister in her big sis's shadows so when her big sister died she thought he won. She got her man and she felt like that gave her a leg up over those that looked down on her. She acted like she liked him before even knowing him cause she was finally going to be "loved".
She also got it in her head that he liked her when they first met, because he was kind to her (had no reason not to be). Basically she’s the type of person who jumps the gun when someone shows her a bit of kindness because of her inferiority complex with her deceased sister (like @Leta Cent said)
Does Larissa actually LIKE Alfonso or does she like the IDEA of him?? I actually cant tell.