There is a reason why we still study Shakespeare: his presentation of universal themes. It doesn't matter that the story took place over 400 years ago, the audience can and still relate to the themes of rebelling against your parents, falling in love with someone for the first time, making choices that affect others in a profound way and so on. These ideas transend time and cultures.

Keep in mind that Shakespeare never would have imagined that his works would end up out living him for generations to come. It's not like he woke up one morning thinking, "Hey I better write a play that all future high school students will have to study long after I'm worm-meat!" Somehow, the Bard managed to tap into ideas that are universial and present them in way that appeals to audience.

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.