Depression and Housing Crisis Caused by Optimism

The Great Depression and the recent housing crisis where caused by excess optimism. I've heard arguments that either using gold-backed money or not using gold-backed money is what causes economic depression. I think these arguments miss the point. Even if money was gold-backed, banks could create notes that are guaranteed to have value in some way, and then they could run into problems if they promise more than they deliver. The essential problem is banks (or lenders of any kind) becoming too optimistic and creating "IOU's" of various forms that ultimately cannot be paid.