See this is the trouble I've always had with Nico, he seems to be a more politician type driver and says things to media that then seem to step over each other. He tends to not be as outright as others. However, I still believe it's about family for him to a degree. His goal was always to win a WDC and perhaps beat Lewis, but I doubt him quitting in general has anything to do with Hamilton. He wanted to win the trophy sure who wouldn't as an F1 driver, and yes it was about that, once he had it that's all he needed to confirm he was one of the best. After that why continue he's already proven it, anything after would be taking away from his immediate family (wife and kids), and how would that be fair to them in his eyes? He achieved his dream, his family put him first and helped him get to his ultimate goal, it doesn't matter who he was racing in the sister car.
Now sure it just so happens to be Lewis Hamilton, a long time rival and once best friend. And media has played it up to be a "me vs him" type thing, rather than a Rosberg fighting against all. Lewis's comment has nothing to do with the general reason Rosberg quit, his comment and Rosberg saying it has to do with Media continuing to ask the questions about their personal rivalry. But the Sport is greater than those two fighting and the sport is the context Rosberg's retirement should be put in. Not the "well he quit cause he knew he wouldn't ever beat Lewis again", that argument seems quite petty.