I think Sophie just had a silly crush on Fitz and I think Fitz mistook their close friendship and cognateness as something more and decided that he liked her. Then when Sophie found out she was unmatchable it was a wake-up call for Ritz
Biana said that Fitz didn't really know if he liked Sophie until they moved into Alluvettre. That's also when they started training to be cognates, I think that Fitz misunderstood their relationship as cognates and he was like 'oH wE'rE cOgNaTeS let's get married!'
