Why?
Glinda cries. Why? Why did he leave her?
They were being engaged. He knew about it. All of Oz knew.
"Fiyero… why did you leave me?"
She knows why, but doesn't want to believe it. She doesn't want to believe that he never loved her.
She mumbles something about Nessarose, use her to draw the Witch out, find Fiyero…
He loved Elphaba, and she will not accept that.
And that is why Glinda the Good found it easy to smile and laugh after Nessarose died…
It was only later that she realized what she had done to her best friend.
"Why? Why did I do that?"
She gets no reply.