Maybe because they complement their partner's strength and firmness. Doesn't matter with the gender, though females tend to be the softer side and males are usually the harder side.
My relatives are a group of emotional people for generations regardless of the gender. But most of the males are tough as rock in character even when they are very sensitive to everything around them and have a soft spot with children. Most of the females are more expressive, yet they can be the toughest and wisest.
I say,it is a great combination even with friends. One man, one woman, one a realist, another an idealist; not necessarily in that order ;=)..