Twilight Sparkle is an extremely important pony to you. After ending up in Equestria, she was the one who took you in. She was the one who taught you what friendship can be. She was the one who treated you like an actual person. Over time, the two of you became extremely close. How could you not fall for her? The problem was, you weren't brave enough to let her know that. It took some time to finally build up the courage to tell her how you feel.
But you were a day too late.