Understanding why America's debt is such a big deal?

I don't quite understand why the National Debt is such a big deal. Call me crazy, call me an idiot, call me what you will, but if I were pressed to name one reason why anyone should care about the National Debt, I can't articulate one. (It makes *sense* that the debt should matter, but I can't intelligently *explain* why it matters.) Can someone help me and/or name a good book or books I could read to bring me up to speed here? Thanks!