Why is my theme library load on every page?
In my theme.info.yml i add under libraries "theme/hover3d".


That's why it's loaded on every page. libraries in the theme info file is: 


  A list of libraries (which can contain both CSS and JavaScript assets) to add to all pages where the theme is active.


To add it contextually, attach it to a relevant render array. For example:

function MYTHEME_preprocess_node(&amp;$variables) {
  // Library will only be loaded when a node is rendered on the page.
  $variables['#attached']['library'][] = 'theme/hover3d';
}


See Adding stylesheets (CSS) and JavaScript (JS) to a Drupal 8 theme for more information.