How does life choose it's nominal temperature preferences?
Here's a link to a brief Scientific American article that presents the results from a study of this question.  Basically, for every $\ce{1^{o}C}$ increase in body temperature, 6% fewer fungal species can survive - so the hotter, the better (the longer you're likely to survive).  The downside of "hot" is the increased metabolic rate required, which translates into more time spent consuming food.  If you were to plot the effect of these two competing effects on longevity versus body temperature you would get something like an inverted (upside-down) parabola.  The study found a maximum longevity at 98.1 F, not far off from the observed average body temperature of 98.6 F.