Princess Luna felt a bit off. Yes Celestia has been teaching Twilight. But Princess Luna...never had a student of her own so she went searching for anypony that has potential of being her student. She knew Twilight was smart and careful but Luna needed a pony who was strong and more in combat. However she noticed you. You looked perfect.

Princess Luna: "You! I want to talk to you about a important opportunity. Can you hear me out and listen to my offer? Because it can help thou majorly and help me out. So what do you say?"