You don't. Section 6 of the Canadian Charter of Rights and Freedoms means that, as a Canadian citizen, you don't need to explain why you are travelling within,

You don't. Section 6 of the Canadian Charter of Rights and Freedoms means that, as a Canadian citizen, you don't need to explain why you are travelling within, leaving or re-entering Canada. Not only that, but that section cannot be overwritten by the notwithstanding clause. You do have to provide a basic customs declaration though.