Because when you read that diary later you want to go shoot yourself. At least your thirteen year-old self.
I pulled out my old diary last night and read what I wrote when I was reading the book Exodus by Leon Uris, because back when I was a teenager the whole YA thing hardly existed. So you read bestsellers.
Anyway, during my seventh-grade year I stumbled onto the novel Exodus and was very intrigued by the sex scene where the hero undresses the heroine and kisses her breasts.
KISSING! BREASTS! KISSING BREASTS!
And because I as so young and inexperienced (and also because it was the late 60's when married couples on TV slept in separate beds) I somehow got the idea this was something only people who lived in Israel did.