It appears that since the beginning of time, physical beauty has been a factor in the attraction between men and women. This attraction caused Cain to envy his brother Abel. He rebelled against Allah's command by refusing to accept his father's advice.
At first glance Cain's rebellion might appear strange, but we should remember that, although man has a pure nature, the potential for dichotomy exists. In other words, he has both good and bad qualities. He can become greedy, covetous, possessive, selfish and even destructive. Man is, therefore, capable of seeking self-satisfaction even if it leads to failure in this life and in the hereafter. The path to goodness lies in harnessing the enemy within him, his baser self, by controlling evil thoughts and deeds and practicing moderation in his desires and actions. His reward then will be the delights of this world and the hereafter. Thus Allah tests us through our divided nature.