You're making sense of Cameron's life backwards. Clearly, lots of good things happened for him for him to become Prime Minister (although I'd actually put it rather differently - for him to have turned out like he did, clearly lots of bad things must have happened).
But there is only a sense of inevitability to these things when you read them backwards. Everything happened for a reason, so it all makes perfect sense. But who knows what would have happened if some of those things had been different? All we can say is that Cameron had a lot of breaks in his life as a result of his family's wealth, which made possible the steps he took to become PM. But of course, we can look at the lives of every prime minister and see the steps and breaks they had. We can say with absolute confidence that every prime minister Britain has ever had benefited from good breaks. We already knew that before looking, of course - look at a successful person and you'll find reasons for their success. Of course.