Young adults! Why do we write fiction for and about them? All they care about is pretty girls and punching bullies! Or pretty boys and standing up to bullies, possibly through punches, as the case may be.
The point stands: your average teenage boy or girl is easily the worst candidate for saviour of the universe, yet we place the burden on them all the time. When someone who isn’t a slave to their hormones could more easily save the day, children burdened with “destiny” have to take on the elements and win.
So too is the case with I Am Number Four, a cynical exercise in teen pandering and film tie-ins: the worst possible person is charged with the most important task. We identify with him because we too go to school and have overwrought feelings for girls! Dive in!