ladygrey
I think part of it is because the characters are well done and because it's a story about powerful emotions and it's honest about them rather than trying to be veiled or mysterious or alluding to things rather than confronting them. It's incredibly sweet in some moments and heartbreaking in others.
And I think a big part of it is that it's not immature the way some young adult fiction can be. The characters aren't petulant or trite - they're angry and torn and incredibly compassionate and strong which makes them engaging and interesting and worth reading.