".. the motivational structures within the human brain were forged by natural selection over millions of years. In his framework, the brain has evolved not to make us happy, but to motivate actions that help push our DNA into the next round. Much of the time, in fact, the brain accomplishes that by making us unhappy. Anxiety, hunger, fatigue, loneliness, thirst, anger and fear spur action to meet the competitive challenges we face."
For the whole story:
If someone truly wants to understand human nature and the nature of morality or ethics, understanding human biology in an evolutionary context is the key.