You could go with the age old idea of it being as simple as magnets, the opposites naturally attracting one another.
Or out of a desire for choice, to walk down two opposing paths simultaneously. Though that's more for the rebels in us, those who like to go against the grain. We like the choice, the freedom to have options, and especially variety.
But for me personally? I think it's because any one thing is bad if it's all you have, if it's taken to an extreme. So I may take the polar opposites together sometimes, due to a sense of yin and yang, to help balance things out a little, find the happy medium in the chaos.