[Kakkou]

Originally Posted by Grifis

I followed the whole movie to find out the relationship between Asuna and Shun and why Shun had to meet her at the beginning. Maybe the answer was there and I missed it. I understand nothing.

Well, you're right about the answer being in the movie.

Spoiler for movie:

If you recall there was the whole deal with the Quetzal Coatl going to the huge canyon to sing out their life story as proof they existed before they die.

Shun was dying by the start of the film by some illness and went to the canyon to do the same thing. He then went to the surface because he's always wanted to see what it looked like and also to hopefully find someone who may have heard his death song. As it happens, his song was that melody Asuna heard at the beginning and meeting her fulfilled his two reasons for coming to the surface.