Because I'm Hot-Blooded, Got a Fever of 103

"Why does our body stay at 37° C?" - Pun 

In our body's quest to always keep that magic state of equilibrium, possibly the most important feature is the temperature that we all know: 37°C[1]. We are all familiar with the disastrous effects of deviating from this immortal number. Too low and you'll develop hypothermia where your important organs begin to break down[2]. Too hot and you'll develop hyperthermia, where you're too hot. Fevers are another form of elevated temperature, which is usually purposefully caused by your body to kill off bacteria (or other bad micro-organisms) due to the fact that these baddies cannot handle the heat like our body can. All of these wonders of science revolve around the magic number of 37°C or, more specifically, homeostasis.

Homeostasis is the process where your body tries to maintain stability while changing to meet conditions that will best help you survive. Body temperature is one of the best examples of this. If you're in a cold room, homeostasis is why your brain tells you its cold and to put on a jacket. If you're in a hot room, homeostasis is why you sweat, which helps cool you down. However, other examples include energy and damage control. If your cells don't have enough sugar to function, your brain gets this message and tells you to eat. If you touch a hot stove, your nerves send messages to your brain to tell you to get away from the painful area. If you didn't have homeostasis, either of these examples would kill you.

But back to the question: Why 37°C? Let's take this in a different direction. Our bodies run a lot of different things all at one time. They think, they eat, they poop, they run, they protect. They really are complex machines, just like computers. When you turn on your computer, it takes a little bit of time to get it started. Even when everything is loaded up and you're looking at the home screen, it's still really slow. That's because it needs time to warm up. Computers operate best at temperatures of 50-65°C. If you run too many programs at one time, the system slows down and maybe even crashes! That's because it's getting too hot. Our bodies are just like that. They work best at 37°C. Too cold and they can't get the blood they need to operate. Too hot and the cells begin to break down.

Not all organisms are like us though. We are of a special group called warm-blooded animals. This means that we control our own temperature through the use of energy (calories from our food) to heat us up, much like a pot of water uses energy from the walls to heat up. But there are other organisms out there that we call cold-blooded animals. Examples of these are frogs, snakes, and fish. These animals don't maintain their own body temperature. Their temperature is always equal to the temperature outside.

Thermal image of cold-blooded scorpion who doesn't hold his own body temperature

Thermal image of warm-blooded boy who always has an internal temperature of 37°C (image is in F)

Cold blooded animals therefore must be much more careful about the temperature of their habitat. If it's too cold, they will freeze to death.[3] These animals often like to lie in the sun to increase their body temperature, which can help with their metabolism. While being cold-blooded sounds like it would totally suck, it does have evolutionary advantages. Constantly keeping your body at 37°C requires a lot of energy. You need to eat and eat and eat and eat just to keep at that temperature. If you're cold blooded, you don't need to waste energy on that silly thing. This means you can live on just a diet of flies and not waste all those calories just to keep you warm. If you're not wasting food energy on keeping warm, you'll probably live longer.

Bonus information: Evolution has put us at exactly 37°C because if our equilibrium temperature was any lower, we would be more likely to have infections. But any hotter would require us to eat significantly more just to get the calories needed to keep that temperature stable.