Humans have become the most common race for a number of reasons. First of which might be that they have been very good at not picking sides when others have fought. Elves are likely come to each others' aid for instance, but humans end up on all sides and neutral, ensuring that at least some proliferate. The race has also proven to be highly adaptable. Others tend to be better suited, but only for a limited range of environments. Meanwhile humans are ever present. And after the collapse of the elven empires, the humans were already around in late enough numbers to fill the power vacuum.
Genetics and Reproduction
Humans have a gestation period of nine months. The children take roughly twelve to fourteen years to reach maturity enough to be fertile, but it is commonly considered they be between sixteen and twenty before they are considered adults. Humans normally live between seventy and a hundred years.
Civilization and Culture
Physical fitness is almost universally considered attractive. Many also appreciate traits resulting from common sexual dimorphism, but distinct breaks from such can be considered attractive simply because of being unusual.