Joe, the security guard, thinks that Natasha reminds him of his wife. This is why he lets Natasha and Daniel on the roof. He remembers how he and his wife used to look at each other the same way that Natasha and Daniel look at each other. Joe's wife died last year of cancer, six months after they both retired. They'd had so many plans for retirement and got to do none of them. He knows that Natasha is right that he shouldn't smoke, but he doesn't believe it matters if he works or smokes to death. Joe watches Natasha and Daniel look at each other and thinks that maybe he'll stop smoking and make new plans.
When Joe sees himself and his wife in Daniel and Natasha, it shows again that history is cyclical and the past constantly informs the present and the future, even if that relationship isn't always clear. It's worth noting that achieving retirement is part of the American dream, which shows that for someone like Joe, presumably a US citizen, the dream isn't necessarily possible for him.