Some reoccuring thoughts...

While sitting through lectures of evolution and evolutionary mechanisms, I've had some reoccurring thoughts:
1) If nature is selecting for the most "fit" individuals (strongest individuals survive to mate= the more children they can have the more "fit" they are), why do we have doctors. If a person is sick, they might die. That's good, because that means the person was weak. The less weak people we have, the more strong people we have to reproduce. Which increases the chances of having stronger descendants. That's good, isn't it? It's nature's "way." So why do we interfere with nature "taking its course?" We are working against the very thing that shaped our species.
2) Along the same line: Why do we care about endangered species? It's natural selection. Humans seem to be the strongest species, so why can't we run over the smaller, weaker ones? We essentially use the same resources, don't we? The less animals there are, the more resources we could possibly have (with the exception of meat, of course). Interestingly enough, the same biologists who are into saving endangered species tend also to be for human population control. Yet again, this is contradictory to the mechanisms of evolution.