Reason 1- Donnie Darko is such a mind-fuck, that you will spend days, months, and years trying to figure it all out.
Reason 2- Frank.
Reason 3- Frank is really Elizabeth's dead boyfriend guiding Donnie through his afterlife with Gretchen.
Reason 4- The Philosophy of Time Travel.
Reason 5- Loop Holes in space.
Reason 6- Jake Gyllenhaal and Maggie Gyllenhaal played Elizabeth & Donnie Darko (Siblings)
Reason 7- 28:06:48:12
Reason 8- The Philosophy of Time Travel by Roberta Sparrow ("Grandma Death") (Link to the book can be found, here)
Reason 9- A kick ass sound track. (Playlist to the soundtrack can be found, here )
Reason 10- You can interpret the movie anyway you want to, there's no specific reasoning behind it.