I think the newness of everything. First kiss, first love, first time, Everything is so like Eiza said raw, and emotional. A broken heart at 17 so different than at say 28. I don't remember every kiss I ever had but I'll never forget my first. *sigh*
I also think it tends to be more about the story and the characters than trying to impress the reader with the language or how smart or crass or deep the author is. I go to writing conventions, and I've met a lot of authors who write to be famous, and not to tell a good story, and unfortunately, many of them still seem to get published. For me, the author should be irrelevant in terms of the story and how the story is told. Meet more than a few with a strong contempt for YA. One commented haughtily, "Well, I write for real publishing houses." I knew I'd probably find his work boring and pretentious, and likely missing a plot.