This movie shows the tiny, mad idea we all had, "What would it be like to be separate from Perfect Love ?"
Shrek has a happy fulfilled life but at one point wishes for something else. Just this one thought is enough to hurl him into a completely different world of darkness where even his wife did not know who he was.
All his attempts to win her love fail until the light dawns on him and he remembers True Love is within. There is nothing he need get from anyone.