Here's why:
* Simple Sentence Definition: A simple sentence contains one independent clause (a clause that can stand alone as a complete thought).
* Independent Clause: The sentence has one independent clause, but it also contains a phrase ("countless hours of sadness").
* Phrase: A phrase is a group of words that does not contain both a subject and a verb.
Therefore, the sentence is not a simple sentence. It's a complex sentence because it includes both an independent clause and a phrase.