Why people always want something in return

She asked me, "why do men always want something in return? ". Then I replied "it's normal". She paused and looked at me.
I could tell that was not the answer she expected. But I was definite. I did not think long before giving that answer because it was obvious.
Firstly, John 3:16: God so loved the world that he gave his only son, whosoever believes in him would not perish.... Even God gave us his son for something in return, which is for us to believe in Jesus and declare him our Lord. Meaning he gave us so we give him our lives in return.
This example is obviously not for a selfish reason because he did it for our best.
So there are two (2) forms of reasons people give help or offer of themselves.
Firstly, for selfish reasons, then for our own good.
Your parents send you to school so you become something in life (it's for a reason). They did it wanting something from you: that is your success.
While, a man can shower you with gifts and good stuff for selfish reasons.
So nothing comes for nothing, there's a law of giving and receiving just as there's the law of gravity. So never expect something for nothing there's always a repay. Either for your good and happiness or for the giver's merit.
God is a jealous God, because he wants you to give him his due. A man or spouse, friend, family could also be because we're made in His (God's) image. So when you observe your spouse not jealous when you're closely entangling with someone he identifies as competition then you should know 'something's wrong' (speaking in whispers).
We always want something whether we know it or not. It might be for your happiness or success; or our satisfaction or Hunger.
The boomerang always returns to the sender.