Why do peanut butter sandwiches become hard and how to prevent that? I've done this with many different types and brands of peanut butter and bread, after a few hours (I typically pack them in the morning as a snack for the afternoon) the bread gets rock hard. I'm curious why this happens, but mostly how to keep them from getting hard ?

The bread is never toasted, I just spread the butter. I've tried keeping it in a plastic bag, in the fridge, outside the fridge, wrapped in paper towel...nothing.
Oooh, I hate that. If you put butter on the bread to create a buffer layer between the peanut butter and the bread it may help. Also, be sure to seal the bread from the air to retard the process of getting stale.

The other main option is to keep the peanut butter in a separate container and spread it before eating.