Reeve and Routh were in two different points of Superman's life, which is why their portrayals are different. Reeve was Superman who had not faced inner darkness. Reeve was a Superman who had not been lead to believe his world was alive, only to go back and find it wasn't so. Nor to come back and find that while the basics of the world have not changed, the lives of those closest to him have. Superman as a character goes through these emotional beats. From time to time he will be depressed about his lot in life, about his being the last of his kind. About the state of the world and how his personal life is. Every so often he will broad and carry the weight of the world on his shoulders.