The most evident explanation of the story is that the various parallel lives the 118-year-old Nemo Nobody claims to have lived are all in the imagination of the 9-year-old Nemo, who is forced to choose between his parents, and thus has to analyze which decision would lead him to a better and happy life. This is why when he finally arrives at the decision, everything crumbles because he doesn’t need them anymore. [They also show reporter’s face as of one of the passengers on the train in which his mother leaves. This proves that the reporter is just one of the projections in his imagination (future world) from his own memory (of the day when his mother leaves).
[Note: one should remember that unborn Nemo was not touched by the Angel of Oblivion, so he remembers everything (from parallel lives and imaginations alike) as well as he can predict future sometimes.]
So, the 9-year-old Nemo, when confronted with a crucial decision of choosing which parent to live with, combines his prediction skills and imagination to analyze each of the paths which his decision would lead. In the decision to stay with his mother, he falls in love with Anna, but their happiness is short-lived; they either separate and never meet, or they get married and he dies. In the decision to stay with his father, he falls in love with Elise, but he doesn’t have a happy future in any of the subsequent paths; he either marries Elise, who is unhappy and still in love with Stephano, or he marries Jeanne but being still in love with Elise. After seeing all possible futures, deaths, loves ,etc, till the age of 118, 9-year-old Nemo finally realizes that it is not necessary for him to choose either of his parents and have disastrous futures; instead, he chooses a completely different path, leaving his parents, and pursuing Anna, the one true love of his life.
Then the final scene, the one in which 118-year-old Nemo, on his death bed, says to the world it is the best day of his life, is actually from the real future. He has had a happy life because he got to spend his happy predicted life with Anna (the love of his life). Presently he is ecstatic and calls it the happiest day because now within a few minutes, because of the Big Crunch, he will be able to spend time with Anna all over again.
As the clock strikes 5:50, he cackles and gets up, awaiting his reunion with Anna.
Akash Sebastian 