Whenever I run "php bin/magento setup:upgrade" commmand on ssh for my website which is running on magento 2.2.2. It restore style-l.css file (pub/static/frontend/Magento/luma/en_US/css/style-l.css) to it default file, means it remove all the editing which I done in this file.

What are the exact reason behind this? How can I stop it?