Characters. For some reason, characters in young adult novels have always been much more relatable to me personally than in adult genres like science fiction or mystery (or even romance or other genres that I have dabbled into). Characters in young adult novels, because of their youth, tend to have pretty unique reactions to different settings compared to more mature characters. Also, perhaps because I've recently left my teens, I see a bit of myself and my friends within the characters. I understand what they are going through (especially in novels that deal with relationships) and I love seeing how things turn out.
Love. A lot of young adult novels that I read include relationship issues or characters falling in love, etc. As a girl, I love reading about love and the idea of falling in love. There are numerous young adult novels that deal with just this. I know that there are a lot of other genres that deal with all of these things, however, young love ( like in young adult novels) in my opinion is more fun to read about. First love and sometimes more innocent puppy love is interesting to me (compared to the highly sexualized love in adult novels like in the romance genre).
There are other factors that add in why I like young adult so much. Novels like The Hunger Games and Divergent can really only be done (or rather, done well) with a younger cast of characters. Young adult brings aspects to the literary world that are unique to it. A lot of which is only interesting in young adult. Again, there are exceptions, but even those exceptions are still not quite the same or do not have the same feeling to them. I guess there is no one reason I love to read young adult novels the most. I just do!