"YA stories feature a young adult protagonist or protagonists and usually focus on that character's journey toward maturity (the tradition of the Bildungsroman.). Learning about love / relationships is an important part of that stage in our lives, so it's not surprising so many writers are building strong romantic elements into their YA stories. I don't remember quite such an emphasis on romance in the books my children read as young adults, so I do think the approach has changed. Within my genre of fantasy, there's been an upsurge of paranormal romance, partly generated by the books, but also reflecting the popularity of this sub-genre with adult readers. There are far more female fantasy writers (and female fantasy readers) than there were, say, twenty years ago, and perhaps female writers are more confident about including a good love story in a fantasy novel.