Because our every thought and act is based on a single resolution to receive as much personal pleasure as possible in any given situation, that and only that constitutes the essence of our nature. Hence, it is simply impossible to even unite the atoms into molecules, maintain an existing form, or develop physical bodies and a human consciousness, without the pursuit of some egoistic goal.
Even if we think that the reason for one’s action is to benefit one’s fellow man, after we analyze the reasons behind it, we will find that there is nothing but a disguised egoistic ambition, a sophisticated form of egoistic exploitation. And because our nature is absolute egoism we are only able to discover and feel external stimulus that correspond to our internal egoistic attributes.
Courtesy of http://www.youtube.com/watch?v=APqTnV0C8SY