Thanks. Oddly enough, it's helped me make sense of Tifa as well. I couldn't understand her actions the first time the team gets to Nibelheim, but it all makes so much more sense once I put it into the context of someone basically having their world ripped out from below them and their whole concept of who they were and who they might be completely shattered.
I figure by the time AC starts, she's figured out who she was isn't important any more, it's who she is that matters, and it's annoying her Cloud can't seem to see the truth in this. Of course, this is also because he's an intravert to her extravert, and he had his self-concept really knocked about by Sephiroth getting inside his head, and also by realising Zack had been in there too. He's not too certain who "Cloud" is any more, so he's needing to be alone and brood and introspect while he figures this out. He really seems to perk up after Vincent explains Geostigma as an auto-immune reaction to the "Sephiroth" geneplex - it's like "oh, so my mind rejected him and my body's doing the same! Cool! So it is me in here all along."
Then again, Vincent is the intravert's intravert, so it figures he'd be able to put things into concepts Cloud would not only understand, but accept.