She may be 18, almost 19, but she is an adult. At that age more people had already had physical relations with someone. And she plans to spend eternity with him. Shes is planning something much bigger and more extreme and more permanent than marriage and sex. She is planning on having her life taken from her. To be immortal. So isn't that a bigger commitment? If shes going to commit to something like then obviously she wants to be with Edward for the rest of time. Why not put an official name on it? Plus its confirming the love they have for each other, showing everyone that they are meant for each other and no matter what will be together forever.
And on top of that, it was so sweet, romantic, and intimate.















