Celestia had decided to become better at acting. It was a skill that could become useful in the future, and also something she wanted to learn. So she found the most expensive, well-regarded, respected, praised and recommended instructor in all Equestria and arranged a private lesson with her.
She was rather surprised when she found out who her teacher was going to be, and so was her teacher when she found out she would be teaching to Celestia.
Things didn't exactly play out the way they'd planned after that.