We should conserve our resources so that we can live life to the fullest. Think of it like this: you and three friends are stranded on a small island. You have enough food for all four of you to survive for a week if you conserve the food and eat a specific portion each day.
If one of your friends eats double their share each day, they’ll run out of food halfway through the week. Then, they’ll start poaching the rest of the food. This is what happens when we run out of a specific resource. Tuna and other desirable fish are often overfished, so other species of fish have to be used instead. The orange roughy, for example, is a fish that was once considered unfit to eat since it had a really gross name.
Of course we must if we want to exist as a species. Even if, selfishly, our desire for one resource is so great that we reject the need for it in other places, or for other peoples, without conservation processes we are condemning the future of our planet to a worse state.
On a simplistic level, if you were isolated but had enough food for a week, would you eat the lot on the first day because of your greed?