Why do clical composers do this?

that is what makes it music. the sudden change of mood or expression. listen to the walton viola concerto. the mood is changed so often, that it's very hard to keep up with. or the introduction and rondo capriccioso (saint saens). they tell a story. and in a book, there is a start, climax, and ending. it isn't the same story the whole entire time. if it did, it would be boring to read, likewise; to listen to.