You are in a heads-up pot, and your opponent goes all-in. You are thinking about calling, and, for various reasons, you decide to turn your cards over (e.g., to get a reaction from your opponent, or simply to share your tough decision to the rest of the table).
In some events your hand is considered dead (i.e., you fold). Why is that? Is there an objective reason why you should not be allowed to show your hand while thinking about your decision?