Eames [talking about the dreamers in Yusuf's basement]: And they come here every day to sleep?
Old Man: No, they come to be woken up. The dream has become their reality. Who are you to say otherwise, huh?
The dreaming people have about forty hours of dreaming every day, which takes about four hours of their time. It may seem absurd but when you can dream about whatever you want, why not? The dream is more important to them than the physical world, and the old man has no problem with that as he points out that what we think of as "real" is very subjective.
Cobb [explaining what happened to Mal]: She had locked something away, something deep inside her. A truth she had once known but chose to forget. Limbo became her reality.
Mal was unable to deal with being trapped in Limbo so she made herself think it was real. What if Cobb had done the same? Would they be a happy couple living in their own kind of reality until their physical minds were dead (which would be who knows how long in Limbo years)? We'll never know.
Mal: So certain of your world, of what's real? Do you think he is? Or is he as lost as I was?
Cobb: I know what's real Mal.
Mal: Not any creeping doubts? Not feeling persecuted, Dom? Chased around the globe by anonymous corporations and police forces the way projections persecute the dreamer?
Whoa, Mal just totally flipped the script. Suddenly, we don't know what's a dream. Think about the Mombasa trip. It was a maze (like a dream) where Cobb was chased by gunmen… just like what happened in the Fischer extraction.
Mal: Admit it; you don't believe in one reality anymore. So choose, choose to be here, choose me.
Mal is telling Cobb the same thing Nolan seems to be visually suggesting to us. If we can't tell the difference between reality and dream, does the difference matter? Mal thinks not and, because Cobb doesn't know what real is it's really a matter of choice. Although Dom is quick to bring up that it's not so simple when other people (like your kids) are involved.