This question has been answered on different plains and many ways e.g. the terms 'homo ludens', 'homo oeconomicus' etc. are attempts to grasp what is human nature. My assumption is: human being is the only one who is able to construct paradoxes an to behave paradoxically. I don’t want to point in the direction of mankind (“We are poluting air although we know it is killing”). I’m talking about really specific actions and stances: Someone is eating although he is full. Someone kills a beloved person. Someone wants to scream nevertheless he keeps silent. Love-hate is another good example to illustrate the idea. Perhaps we could consider this idea without recurring to a simplyfying rational choice-concept or could or nullifying neuropsychological or biological approaches.