Saito: Do you want to take a leap of faith? Or do you want to be an old man, filled with regret, waiting to die alone?
Saito and Cobb repeat this back-and-forth three times. It's how Saito hooks Cobb into doing the job, it's what Saito says to Cobb as he's dying in the dream, and it's how Saito and Cobb remind each other who and where they are. The fear of regret— regret of time poorly spent—is what drives Cobb and Saito to come back to a world that matters to them.
Mal: Remember when you asked me to marry you?
Cobb: Of course I do.
Mal: You said you had a dream…
Cobb: That we'd grow old together.
Did they grow old together? It's interesting that Cobb believes they did when he's the one that values the physical world. Maybe their time as gods in Limbo did matter. They were both essentially themselves, and fifty years is a long time. It must have been difficult waking up as young people.