At the moment, Castiel did not have the patience to debate with Sam. He had learned from past experience that Sam Winchester had the capacity to be exceptionally tenacious. "Dean's right, Sam. Knowing your future could only impede important events."
Part of him disliked assuring Sam that his tumultuous future was the right one. He wasn't even sure himself. His second thoughts appeared in a sidelong glance to Dean. It was always better to play it safe with temporal displacement.