A very basic answer is because the thyroid gland produces hormones that predominately regulate OTHER organs and organ systems. If the thyroid produces too few or too many hormones, then this will effect the organs and organ systems will not operate normally.
I was diagnosed with Graves' disease in high school. Here are some resources to help you understand thyroid issues better:
The Thyroid Sourcebook
-- I have found this to be the most helpful book. It's widely available in public libraries. http://www.amazon.com/Thyroid-Sourcebook-5th-Sourcebooks/dp/0071597255/ref=dp_ob_title_bk
A few websites...
-- http://www.endocrineweb.com/conditions/thyroid/your-thyroid-gland
-- http://video.about.com/thyroid/Thyroid-Disease.htm