The Frog

Brace yourselves, everybody, this one's gonna be random.

So if you have a frog and drop him in really cold water, he immediately goes "OI WOT" and jumps immediately back out, as he finds the freezing water to be most disagreeable. Replace the cold water with some near boiling, and he will issue the same proclamation and leap out almost instantly. In either of these extreme situations, the frog is instinctively aware that, if he remains, the water will kill him.

People are like this too; we have a highly-evolved intuitive sense for danger. That's the human advantage in evolution, and the key to why our seemingly physically underwhelming species has lasted so long. We may not outrun the lion, but we're aware of the signs that lions are near, and to a certain extent we have an extra, inexplicable sense for danger. To call it the sixth sense is inaccurate (we have far more than five senses), but the term serves to describe the phenomenon. I don't need to give you examples of this because everyone has experienced the sensation; nothing is outwardly wrong, but you just get a feeling.

In this way, humans share an animal's sense for painful situations, as well as an anticipatory ability to avoid them based on their precursory signs. That's fine. Consider, however, a different scenario.

If you drop the same intrepid little amphibian into a pleasantly warm pot of water, he won't react to it at all, and think something like "ah yiss, dis is niiiiice." That's his natural temperature, after all, and even we mighty humans can relate to the enjoyment of mucking about in warm water. So, he (and we) will swim around aimlessly and enjoy the experience.

Here's the problem: what if you turn the temperature up? Not a lot all at once, mind, but incrementally, a little at a time.

This is a strange paradox. Once the frog gets used to the starting temperature of the water, a little additional heat just feels like the pleasant warmth from earlier. He adjusts again, more heat feels nice again. This can continue for a while, until the temperature gets to the point of fatality, at which point the frog will again think "ahh, pleasant warmth" and, as they say, croak, having never realized how hot it was getting.

"What a foolhardy amphibian, to not notice he's being boiled!" we mighty humans proclaim. "I would surely get out of the water."

Well, that's the issue, isn't it? We humans are even more adaptable than frogs as far as temperature is concerned, having our own internal mechanisms to adjust our bodily function rather than being totally subject to the whim of the environment as cold-blooded creatures are.

Getting used to a temperature is easy, and you can motor along through life at basically any temperature. Spikes in temperature (dangerous situations) are obvious and can be managed; we're smart, generally, and we have fair bit of knowledge about how to not die. Consider, however, the frog who never notices the temperature slowly rising around him because he's so acclimated to it. When you suddenly feel something boiling up inside you and the temperature is spiking, stop and ask yourself:

"What temperature have I been at?"

TL;DR Being used to something =/= it being a healthy state.

P.S. I came to this realization after burning myself in the shower. Just in case someone thought I was being insightful or something :P