Here's why:
* A complete sentence needs a subject and a verb. "He smiled" has both a subject ("He") and a verb ("smiled").
* A complete sentence needs to express a complete thought. "He smiled" doesn't tell us *what* he smiled at, *why* he smiled, or *what happened after*. It leaves the reader wanting more information.
To make it a complete sentence, you could add more context:
* He smiled at the joke.
* He smiled because he was happy.
* He smiled and walked away.