Here's why:
* Gerunds are verb forms ending in "-ing" that function as nouns.
* "Partying" is a verb form ending in "-ing" (the base verb is "party").
* "Partying" can function as a noun in a sentence:
* "Partying is fun." (Subject of the sentence)
* "I enjoy partying." (Object of the verb "enjoy")
Let me know if you'd like more examples or explanations!