It is the optimum temperature for the enzymes in our body.
Many of the processes in the human body rely on enzymes; proteins that catalyse biological reactions.
As enzymes are proteins, it is easy for their structure to be changed and function affected. At very high or low temperatures, the enzymes in our body can be altered. Their shapes are changed and we say they have been denatured. This makes it harder for them to do their “job” and the reaction they catalyse (speed up) will be less efficient.
Enzymes have an optimum temperature too. At this temperature, they are at their most productive (as shown by the graph). For the enzymes in our bodies, optimum temperature is around ##37^@## .
Another reason for maintaining body temperature at ##37^@## is that if the body is too hot, it will lose water more easily (it will evaporate) leading to a higher risk of dehydration. It also take a lot of energy to heat the body. Maintaining a high temperature would mean the body had to have a higher metabolic rate.