Cobb: [about Mal] She had locked something away, something deep inside her. The truth that she had once known, but... she chose to forget. Limbo became her reality.
Ariadne: What happened when you woke up?
Cobb: To wake up from that after, after years, after decades... after we'd become old souls thrown back into youth like that... I knew something was wrong with her. She just wouldn't admit it. Eventually, she told me the truth. She was possessed by an idea, this one, very simple idea, that changed everything. That our world wasn't real. That she needed to wake up to come back to reality, that, in order to get back home, we had to kill ourselves.