There was a lot to this book, even before Zack and Nikki meet. They have both had a rough few years, and could not fathom the twist fate had in store for either of them. Nikki received a shock after her mother passed away and now she finds herself in California getting to know the aunt she never knew she had, and looking for a sister she also didn't know existed. Meeting Zack was a bright point in an otherwise confusing, emotional journey. 
 I loved Zack's little notes to NIkki. Based on his mental state when they met, these notes give a glimpse at his feelings (for Nikki) long before he can verbalize them. He is such a good man, that his pain and guilt was hard to watch for a long time, but his journey back to himself was wonderful. 
 Growing up with a paranoid, bipolar mother, trusting strangers was difficult for Nikki, and as far as she knows her aunt is one of those strangers. Over the course of Nikki's search for answers, while I would have gone about it differently, I understood why she made the decisions she did. Grief and shock plays out differently for different characters and she did what she thought was safest and the fastest way to get her answers. 
 Neither was looking for a relationship, and at the time Zack was about as closed off as a person could be, but the pull between them was too strong for either to fight. I loved the idea of destiny pulling them despite their circumstances. The spark between them was immediate and inevitable. 
 The title concept evolves to mean several things to different characters over the course of the book. In some ways, every major (and a few minor) characters face being left behind, whether they realize it or not. And the implications from being left play out differently for each person and makes this a dynamic theme throughout the story. 
 This was a quick read with characters that were easily likeable. There was a great mix of vulnerability and strength in both Nikki and Zack. While some of the answers to Nikki's search were clear long before Nikki learned the truth, I found it interesting how those who did know the truth handled the teens. 
 I was gifted a copy in exchange for an honest review. This review is also posted at Reviews by Tammy and Kim.