Reasons? Its pretty clear and obvious to me, but possibly not socially correct to say.
Biology. In evolutionary terms we're still the same creatures as our neolithic ancestors. The clear division of roles by sex, that you see in most other animal species, are also baked into our brains. To the degree we cannot fill the roles that our brains tell us we should be filling, we feel useless, unfulfilled, and unhappy.
Women feel unhappy as they live out their childbearing years without a male partner, children or without fulfilling the role of mother and nurturer.
Men feel unhappy as they live out the prime of their lives without children and a female partner and fulfilling the roles of protector and provider.
Obviously there are exceptions, and compensating mechanisms. But it's not that complicated.