Rosberg has to figure out how to battle it out. During the US GP, one of the announcers (Coulthard IIRC) commented that if Lewis gets passed, he instinctively goes wheel-to-wheel and battles to get his position back. He can change his lines instantly to an attack strategy. Rosberg doesn't think like that. He's super fast, as his qualifying results show, but he can never beat Lewis in a fair fight because he can't attack/defend as well as Lewis. In Turn 1, he didn't need to crash into Lewis to teach him a lesson; he needed to back out earlier and undercut Lewis instead, as Lewis has done on him in the past. Lewis has won the WDC the last 2 years because Nico doesn't deserve to win it.
Off-track, I'm not a big Lewis fan, but there are few drivers more exciting to watch on track.