Do I always have to be grateful for different things every day?

No, but it can be a good practice to stretch for a new thing, allowing more aspects of your life to be mined for gratitude.
Or if not a new gratitude, I try to find a different aspect of the constitent thing (eg. “I’m grateful for my dog” can also be, “I’m grateful for the companionship of my dog,” or “how pleased he is to see me when I get home.”)