Depends on what you mean by tough—if you mean chewy and crisp, that is desirable for a lot of artisan breads. If it happens on soft sandwich breads, that’s another story. A higher baking temperature will produce a thinner crust. Also more fat in the dough, or brushed on the crust before and after baking will make it more tender. You can also avoid spraying the loaf directly with water, which can make the crust chewier. It might be something simpler though like using too much flour in your recipe, which can make the bread dense, tough, and dry.