Still thinking about the end, having closed himself off to others and not letting anyone get too close after the beginning's turn of events, during the course of his journey Joel lets down his guard and gets close to Ellie (perhaps too close).
With nothing or no one else in the world left for him, by the end he was willing to fight and go to any lengths to stay with her, she is what he fights to stay alive for now, his reason for living, without her he has nothing. I remember him saying as they arrive at Saint Marys 'im not leaving without you' which is a complete turn around from the way he was at the start.
By the end of the story, the two characters personnalities had completely flipped.
The best way to find out if you can trust somebody is to trust them.
Ernest Hemmingway
