how can we lose ourselves

it was a good movie but made me very sad. i thought of my grandfather who lost himself and finally let go some years back. but now i think of my grandmother and for the first time in a while i'm crying. she is only a shell of herself. my dad said when he looked in her eyes there was nothing looking back. we chanted with her and said it was ok to let go. it's so hard seeing her and thinking of what she used to be like, how active she was, and how i know she would have said "i hope that doesn't happen to me, that would be the worst, forgetting who you are and who your family is" but she has. i don't understand it. how can we lose ourselves?