Hm. Now you've given me something to think about. I've always associated childlike exuberance with these two but I hadn't given curiosity any thought but you might have a point. I've just always viewed Jack as 'molding' shall we say, Ianto into a good, strong, capable man. Ianto holds a mirror up to Jack and makes him want to continue to be a better man. Ianto continues the process the Doctor started for Jack.