Maybe I'm just overcomplicating things, but this seemed like quite a philosophical book to me (in a Patrick Ness sort of way, of course)
I think that Patrick Ness showed in a weird kind of way that actually, every story counts. Not just the main character, Every single story has an interesting story to it. I thought that was quite clever.
Although, I might be completely wrong with my interpretation, I mean Patrick kind of showed at the end of the story that things can mean so many different things to so many different people, and sometimes it might not mean anything at all.

okay so here is where I start blurting SPOILERS if you don't want to read those....

I don't know why, but I had some trouble relating to the characters, Henna in particular. She just bothered me, like she didn't know what she wanted, even though the tattoo and all was supposed to signify her choosing her own person (I think?) but she still confused me. Like, her and Mikey when she's in the car, saying "Um no what are you talking about you don't love me." then like a few chapters later, "oh well I had a brush with death and that made me realise that actually, I've never kissed you, which I think I'd like to do before I die. but, you know, unless this kiss is REALLY good, I'm still into this Nathan guy." and then when the kiss IS really good, "oh wow I think we were made to be together let's get together." then when they're together "do you know what, now that we're actually together, after all these years that I've been dating other guys but secretly wanting to kiss you, I realise that we're just meant to be friends. please excuse me while I explain that Nathan is actually gay and I get back together with my old boyfriend." ... why?!
so anyway, that was probably my main problem with the book. Also, I think Jared was the most random, but still relevant, character I have ever read about.

in conclusion, good book, but not 5 stars (well, for me anyway). probably 4.4-5 stars. Definitely enjoyed it.