How do we re-use Paragraph items? Paragraph items are only available to their parent entities. But is there any known way to make it possible to re-use existing Paragraph items?
Paragraphs is focused on reusing layouts not content. The most obvious implication is on revisions.

In a standard reference field you reference the latest revision of the target. When you roll back the parent the target keeps the actual content. Paragraphs on the other hand keeps track of the revisions and allows you to rollback an entity including the content of the paragraphs that are owned by the entity. And that's the problem, if Paragraphs would allow reusing, how a rollback should be handled for paragraphs that are shared among multiple host entities.

So Paragraphs is by design not a tool for reusing content.

But there is good news, there will be a library entity, which will make this possible:


  Implement Library entity type
  
  We want to make this separate entity type:
  - Autosync with bundles + fields of Paragraphs
  - Still add a label (plus more administrational fields if needed for managing them).
  
  A reusable paragraph will be typically stored as a clone of a previous
  paragraph. It is NOT a composite of the original. Reusable paragraphs
  have an own separate edit form and can also be creted independently
  through the Add form.
  
  A user can click the action "Add to library" that makes a paragraph
  reusable. This clones the paragraph and the original host entity will
  have a new paragraph instance added that contains a reference to the
  effective reusable paragraph.
  
  The paragraph type "From Library" contains nothing but a entity
  reference to the reusable paragraph. Initially, using autocomplete
  widget here is fine.


https://www.drupal.org/node/2843917