Donnie Darko Question?

Frank basically is what made Donnie deviate from his real destiny (getting killed from the jet engine), and so from the moment it fell, he was living in a tangent universe ( a loophole in his fate). This tangent universe is very unstable, and eventually his own girlfriend is killed. After he makes the realization that by avoiding his own fate, he affects the fate of others (ex :frank, gretchen) he decides to go back and carry out his fate the proper way. He laughs at the end because he figured it all out.