I hate to hear an adult say to a teen, "You think life is hard now? Just wait until you have real problems." Whatever problem a teen has, it is real. When something is the hardest thing you've dealt with so far, it's a hard thing. It's a big deal. It shouldn't be judged in comparison to anything else and minimized, especially not by someone with the advantage of far more life experience.
What matters in YA is everything. Everything teens face and struggle with. Everything they cherish and celebrate. Everything they know and everything they don't know. There is no checklist for what matters.