Theories. Most of them don't even know what they are up to but that doesn't stop them from existing. Yes, that's one of the side-effects of thinking. So here's my two cent about something that might explain why some survive while others don't. It can't be proven scientifically but that's also the reason why it's worth a thought. Here we go: What if there's a fixed quota when it comes to life (in terms of seconds or minutes or days or weeks or months or years or decades or...of course)? In other words, our existence as a species can't be stretched beyond certain point. All in all, just a matter of maintaining balance. Children take birth; about 3,75,000 on a daily basis. People die; about 1,70,000 everyday. What if there's a math hidden in there somewhere? Yea, yea, yeah. At any given point of time, more people have died on this planet than those who are left behind. But what if some people die and their could-have-been age is transferred to somebody else's account? Like inheritance of existence. Or something close. It's like saying food never goes to waste because even microorganisms also have dreams—and micro-stomachs to fill. What if imbalance works in mysterious ways. We just haven't figured out yet. And hopefully, we won't. Ever.
NB: Just a thought. No need to kill me for wasting your not-so-valuable time.