There are a lot of different reasons why Romeo and Juliet is considered important. For one, if you look at the language that Shakespeare employed, it is beautiful. Lines like, "My only love sprung from my only hate!
Too early seen unknown, and known too late!
Prodigious birth of love it is to me,
That I must love a loathed enemy (I, v)," flow with such imagery and imagination.
I think the reason why it is taught so much in high schools is, as you say, its themes. Themes like love and lost, and going against one's parents is accesible to students. It might be hard to understand the language, but its easy to understand the sentiment. Its easy to see why Juliet is worried about her parents finding out about Romeo. The themes and the language make this play spectacular. But if you don't love this one, try another Shakespeare play to see if that one resonates more.