Why is the average human body temperature 37 degrees Celsius?

The human body runs on a large number of biochemical processes. Chemical reactions happen faster at higher temperatures, so the body needs to have a certain amount of heat in order to function in an optimal manner. However, it is dangerous to have too high a temperature because at some point, there are going to be unwanted chemical reactions, disrupting the useful ones. So there is a balance, which for human beings is about 37o C.
The enzymes and chemical reactions in the human body is optimal at 37 degrees Celsius. If it is too high, enzymes will denature and fail to perform its necessary function. If it is too low, the reaction rate is too slow.