When one steps into the outside world, one understands whats business. You give me this and in return you would get this. Indeed it is logical equilibrium and common sense that one gets an equivalent amount for his investment.
Its the same with people, you would do something for a person only when you know you can get an equivalent favour sometime in the future. Time, money, love and attention are too precious to be wasted on traitors. Yes, step into the real world and you learn this.
I completely agree with the rule, I have understood it through experience. Those who dont follow this simple common sense are usually exploited and they deserve it in return for their own stupidity.
And yet I have knowingly broken this rule again and again. I have known other people like me who do the same thing. People who would invest a lot of every kind indiscriminately, not checking if the person getting those favours deserves them or not. I have been and I have seen people burnt because of it.
I totally agree that its stupid. But I think the world would be a really poor place without such stupidities.