Chocolate evolved the day when you were trapped in a cave with him. He tried to move the boulders, but he struggled. Out of sheer stubborn will to help you, Chocolate finally evolved into Pangoro! He helped you get out and you both ran back to the Pokemon School. Everyone was shocked, not at all expecting Chocolate to actually evolve. Chocolate was happy with himself, you were very proud of him too.

But Chocolate is still nervous about battling. Even though he's proud of himself for finally evolving, he's still scared about battling. Either because he doesn't want to get hurt, or he just doesn't have the pleasure of hurting others. Probably the second one.