Whether you know it or not, every document you create in Word starts with the template named Normal.dotm.
This template includes the default styles and behavior that determine the look of each new document you create.
You may modify this template to avoid resetting something every time you open a new document. BUT, beware that if Normal.dotm is damaged or moved, Word will create a new version the next time you open Word, but it will not include the customization you have previously made.
The same thing could occur when you upgrade to the next version of Office.