Why journal in a book? Why not loose pages? It is kind of like your life. Waking up every morning, we have to go back to it just the way it was left the night before—being forced to look at mistakes, successes, hopes, hurts—and go on to the next page. Even when we don't want to. Even when we just want to go out and buy a new book.
We keep going. Hauling it around like a bag of rocks and wondering what in the world we were thinking—buying a book this size or jumping into that relationship or taking that job too quickly. It all comes out the same. The bottom line is, we all have to get up the next day and face it.
Loose pages can be shuffled, tossed, crumbled, prearranged...its hard for them to represent something substantial. Often they end up as preconcieved, acceptable pages saying all the right things, written for others to see and smile and tell you how wonderful you are.
Loose journal pages aren't heavy and can simply be tucked into whatever you are carrying. They can also easily be blown out of you hand in a moment.
A book has substance. Concise pages bound together so tightly, they, like your daily event, make you who you are.
So I think that is what I was thinking about when I was questioning as well. Although I had not conceptualized the answer yet. I am not by any means saying that it is unacceptable to journal on loose pages, just found reasoning to support the thoughts I was thinking for myself.