Here's why:
* Predicate: The part of a sentence that tells us what the subject does or is.
* Complete Predicate: Includes the verb and all the words that modify the verb (adverbs, prepositional phrases, objects, etc.).
In this sentence:
* Subject: Our friendship
* Verb: has lasted
* Modifiers: a long time, despite differences
Therefore, the complete predicate encompasses the entire action and its circumstances.