why is my bread too tough?
Magnus Nordlander's answer was good.  I would add one more thing:

You can also get a lot of extra toughness if you over knead a dough.  If you are using an established recipe that works for other people, I would cut back on the mixing and kneading and see what kind of results you get.  If you don't knead enough, you won't get the airy texture of the crumb because the gluten won't be developed enough.  

If you bake a lot, make changes to your kneading time and take notes about how it affects your final result.