Algebra..

Equation 1
Human = eat + sleep + work + enjoy
Donkey = eat + sleep
Therefore:
Human = Donkey + Work + enjoy
Therefore:
Human - enjoy = Donkey + Work

In other words,

A Human that doesn't know how to enjoy = Donkey that works.

Equation 2
Man = eat + sleep + earn money
Donkey = eat + sleep
Therefore:
Man = Donkey + earn money
Therefore:
Man - earn money = Donkey

In other words

Man who doesn't earn money = Donkey

Equation 3

Woman= eat + sleep + spend
Donkey = eat + sleep
Therefore:
Woman = Donkey + spend
Woman - spend = Donkey

In other words,

Woman who doesn't spend = Donkey

To Conclude:

From Equation 2 and Equation 3,
Man who doesn't earn money = Woman who doesn't spend

1. So Man earns money not to let woman become a donkey!
2. And a woman spends not to let the man become a donkey! 

So, We have:
Man + Woman = Donkey + earn money + Donkey + Spend money
Therefore from postulates 1 and 2, we can conclude

Man + Woman = 2 Donkeys that live happily together.