why is my bread too tough? trying to get the hang of making good artisan bread, and my last loaf tasted great. But it stayed flatter than previous efforts, and was very tough.

I am doing my best to understand the science as clearly as possible, so I want to know what part of my process I should look at tweaking to get the results I want?
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.