In a nutshell, the song tells us about someone (a girl) who is crushing on her boy. She adores the boy so much that she always remembers about him everywhere she is. She remembers about the 'crazy things' that her boy have ever said and 'crazy things' that they have ever done together. Well, she actually does not TRY to remember it all. The memory just comes to her without she trying to remember it. I believe you who ever fall in love know how the feeling. It's true, wherever you are, whatever you do, your mind is full with him/her you love so much. Yups, I know the feeling as well.