The name Journey of the Spark came from a line that Twilight says in the second episode of Season 1.

The movie focuses on Twilight’s emotional journey into adulthood. Its made apparent in a multitude of places in the Friendship is Magic continuity that Twilight is the force that connects the Mane 6 together as well as the Elements of Harmony. So in a way, Twilight is the spark.

Hence the name: Journey of the Spark