I would guess that it's because she has been waiting for this moment for a long time and when it happens from a guy she really loves she is So happy :)) xxx
What does it mean when a girl cries after she has been asked to be married?
Crying is usually just an outlet for an excess of emotion. Whether it's anger, or sadness, or, in this case, joy.
Usually they are tears of joy/happiness. I cried when I was asked by my husband 21 years ago.