Lovers have their reasons. To Twilight Sparkle, the reasons she loves Celestia are perfectly obvious. And to Princess Celestia the reasons why she can't love Twilight back are equally obvious.
Twilight never became an alicorn princess and instead lost her life as a result of Star-Swirl the Bearded's unpredictable spell. But being dead doesn't prevent Twilight from visiting those she cares for...