I don't think it was sudden at all.

Squall is simply desperate to not let his emotions take a hold of him from the start of the story. This is simply because he doesn't want the pain that he believes is inevitable with any human relationship. As such, he is obviously confused by the strong feelings shown to him by Rinoa. He clearly does not know how to handle it.

I don't think it's as cliche as he doesn't know what he's got until it's gone. I think that it is just that up until this point in the game he does not have the motivation for action. He can't stand the idea that Rinoa would be incarcerated forever in Esthar and is pretty much forced to do something. After this, he still doesn't completely open up. There's no remarkable transformation into a happy loving Squall. He just had to something to free Rinoa.