My personal view on it, which I cannot support with as many facts as I'd like, is that Cloud and Tifa knew each other at least decently well- they were next door neighbors, after all, and it's very difficult not to get to know those people very well- but that Tifa sort of took his presence, and him, for granted, but that after he made his declaration and went off to join Soldier, she realized what a ' good thing' had been under her nose the entire time.