Here's why:
* Gerunds are verb forms ending in "-ing" that act as nouns.
* In this sentence, "laughing" is the action being discussed, and it's the thing that is "good for a person."
* The verb "is" is the main verb, linking the subject "laughing" to the predicate "good for a person."
Therefore, the gerund "laughing" is the subject of the sentence, performing the action of being good for a person.