After seeing what happened when she accidently changed her friends destinies, Twilight is curious about about the 'what if'. Like, for example, what if Luna had ponies a thousand years ago who noticed her.
Raising the sun isn't an easy job. Celestia knows that better than anypony. It requires vision and creativity, and she can only do her best work when the inspiration strikes her. But, as is true with all good art, nobody else cares about that.