A lot has been written about this movie on the interwebs, and theories about the mechanics of the plot abound. I have no new theories about who was dreaming when, how limbo works, how effective Cobb's totem is, and all that other stuff. The first two or three times I saw the movie, that's what I focused on until I accepted that whether Nolan intended it or not, the whole thing was a meta-dream experience for me. I can't make total sense of my dreams, and I couldn't make total sense of the movie. Instead, I'd like to look at the movie as it examines my favorite subject—death.
For anyone writing in English, death and dreams are inseparably linked. That's just part of our literary heritage. There is something of death in dreaming and something of dreams in living. "Come heavy sleep, the image of true death," writes John Dowland's anonymous lyricist. "Our life as a dream, our time as a stream glides swiftly away, and the fugitive moment refuses to stay," says Charles Wesley. Conrad writes, ""What dreams may come when we have shuffled off this mortal coil must give us pause," says the Dane.
Think about it, when the characters died in a dream, they either woke up or plunged into an infinity of thought and further delusion.