Dragon Ball Super: Destruction

After training for a whole year, Trunks realized one important thing: Saiyans don't transform just because they want to or train for it. The only exception is Super Saiyan God, which is just a transformation based on learning god ki.

So Trunks asked, "Why do we Saiyans transform?"

Whis replied, "Trunks, have you ever wondered why Saiyans never transformed for hundreds of years, and why you all started transforming after Goku did? Well, you surely get stronger with training and evolve your transformation. Like with enough training, your Super Saiyan form can be evolved, but that is not a transformation.

The main reason for transformation is need; it is the need to survive. Your ancestors fought different races, and when they found themselves in difficult situations, they transformed into Ōzaru. With Frieza, they just accepted they couldn't win, so they never desired to win. But for Goku, he needed the power to save his friends and son, to save the planet. So that need fulfilled as a transformation, but there is a catch: your body must be strong enough to handle that power.

That's where training comes in, and all this time I have been training you to improve your base form and evolve the Super Saiyan transformation. Your body should be ready if you need more power, and on top of that, the base form is the basis for all transformations.

I have already trained you enough to fight some stronger foes, but the need to survive will not come from just sparring. So, you either stay as you are or travel to the universe and fight stronger foes. I will suggest you join the Galactic Patrol for now; you will be fighting some strong foes there. Think of this as another part of training."

Trunks decided to do as Whis said, so Whis took him to the Galactic Patrol HQ, where Trunks met Merus for the first time, and then Whis left.

As per Merus' request from HQ, Trunks will be working on high-difficulty cases with Merus, as Whis told him to push him hard. We all know Merus is an Angel Trainee, but Trunks does not know this, and they start working on some difficult cases.