Twilight loves Celestia, though she had never had the courage to tell her until today. But it seems that forces have aligned to prevent her from being with the one she loves. Twilight fights against all odds to deliver the letter she had struggled so long to write. However, it may turn out that neither she, nor Celestia could possibly be ready for the consequences of their relationship.
Because being immortal does not mean being invincible. And a Princess is no more immune to pain than a pauper.
... especially in matters of the heart.