Romeo and Juliet is about the folly of youth: how everything is so new and fresh and intense that you end up doing foolish things like stabbing yourself when that boy you met a week ago seems to have died.
West Side Story is a little kinder towards its protagonists, but they're all definitely very young, and have the passions and impulsivity that comes with it. They live in the moment. That's what makes Tony and Maria pledge their lives to each other two hours after they meet, or why the Jets and the Sharks are so happy to prove who's boss once and for all over and over again. They're all full of energy and yet they don't think things through like they should: a tragedy that always hits the hardest when young people are involved.
Chew on This
The young people in the film get nowhere because they're foolish, impulsive, and contemptuous of authority.
Tony and Maria's youth is what allows them to love and hope.