People like to think that the world is mostly good.  The virtues of mankind outweigh any problems.  However, the concept of altruism - doing good for no personal benefit - becomes unachievable due to ulterior motives that most would not consider.
Bill Gates has been lauded over his philanthropy and altruism.  After starting the multinational corporation Microsoft, Gates stepped down from his position of CEO in 2000 and devoted his life to philanthropic endeavors.  His Bill and Melinda Gates Foundation donates large sums of money every year to charities and scientific research programs.  Although his actions seem almost entirely based on the benefit of others, is Bill Gates really an altruist?  When Gates does good for others, he could have several motives.  If the situation arose that he could help someone else, he could be persuaded to act based on the good feelings that gets from helping others.  The pride that comes from this could be reason enough to continue helping people.  In this case, Gates would be, for the most part, acting purely for his own self-interest.  Also, in the case that he decided not to help, he could be haunted with regret for not acting.  The fear of guilt is also reason enough to continue helping others.  Bill Gate's philanthropy, therefore, cannot be described as altruistic, because it benefits him, helping him avoid guilt, and rewarding him with pride.
Another reason for seemingly altruistic behavior comes from an evolutionary standpoint.  If someone saves a child from an oncoming car, they could be acting altruistically, but evolutionarily, they are acting to preserve their species.  Saving a child will keep their genes in the gene pool.  In order for a species to be successful, every member should be protected.  In this case, saving the child would not be just a good deed, it would be done to better humankind.
Overall, in situations where one's actions could be described as "altruistic", there is always another beneficial product that comes with the act.  Because of this, no act can truly altruistic, because the word implies that no personal benefit comes to the individual.