I didn't get the movie "donnie Darko"?

okay so i finish the movie donnie darko and i think about and - Frank called him out that day in the begining in the movie, because of that it ended the world; so he has to go back in time to make sure he dies.- Okay Now Wtf? why would the world end and how did he go back in time at the very end, and why donnie? why not somebody else?