As diarch of Equestria it is Celestia's responsibility to protect her subjects from harm... even when it means losing her sister in defeating Nightmare Moon. Eternal night it was confirmed would have brought cold and starvation, Equestria would have died. It was a decision Celestia and to make... and was forced to live with for a 1000 years. The guilt, the pain, the loneliness all this Celestia had to bare until she was able to help make sure her sister could be saved later on.
Well when Lauren Faust was still with the show she confirmed that eternal night would have essentially meant the end of Equestria. Also as it was shown in Princess Twilight Sparkle, Celestia was in tears after gaining the Elements and during the final confrontation with Nightmare Moon. Lastly, if you allow the comics to be considered as canon, it is show that Nightmare Moon was Luna taken control by a dark entity.
Yet despite all this it is your choice not to be convinces, it is a good thing you don't frequent MLP Forums as I play Princess Celestia there.