Because the boy is insanely talented and has just lost his way. A spell at Madrid could do him some good and make him grow up a bit. If he comes back here after, I'd be delighted. Could be a great deal.
Still it's not like Madrid to make such signings is it? They buy players who are impressing currently, not in need of a refinding themselves
(Agree he's a great player/potential - I'd sooner him stay than the manager tbf)