Act only according to that maxim whereby you can at the same time will that it should become a universal law - Immanuel Kant
Even the body within which individuals treat each other as equals [...] will have to be an incarnate will to power; it will strive to grow, spread, seize, become predominant—not from any morality or immorality but because it is living and because life simply is will to power - Friedrich Nietzche
The needs, desires, and interests of another person can never be more important to me than my own. This is a simple, ineradicable fact of reality. I may indeed feel sympathy for their pain and suffering, and likewise happiness for their joys and triumphs. But when another is tortured it is not me that is tortured; when another goes hungry it is not me that goes hungry; when another succeeds, it is not me who succeeds; when another person dies, it is not me who dies. Only I can suffer my own pain, satisfy my own urges, glory in my own accomplishments, and delight in my own love. Any reflections I feel of another's happiness or discontent is just that, a reflection, a shadow - a feeling perhaps more acute, perhaps more powerful than some other less intense pleasure or irritation, but certainly never more so than what I would feel if I were in the same situation! I cannot know what it is to be them, nor can they know what it is to be me. My reality is this moment, this decision, this instant of life; it begins with memory and ends in death.
Morality can be thought of as the distance between 'the best possible world', and the world as it really is. Within the sphere of my reality, I can control only my own actions and the consequences thereof. Rationally, my primary motivation (my will, one might say) is to try and predict which courses of action and reaction will maximize my own happiness in my life, and minimize the degree of suffering I will have to undergo to achieve this happiness. In life, there are many different sources of happiness: the myriad physical pleasures, social and emotional fulfillment, the gratification of achievement, the satisfactions of liberty and security. There are also many different sources of suffering: physical pain and want, frustration of desires and ambitions, fear for safety and security, the aforementioned sympathy for another's pain. I am a limited, evolved being, a sack of meat and chemicals; I have only circumscribed knowledge of reality, and a makeshift faculty of reason too easily overwhelmed. In order to best carry out my will, I must therefore always strive to increase the former and strengthen the latter. Even so, I will inevitably make mistakes, decisions based on limited information or mistaken beliefs or momentary humours. I must own and acknowledge my mistakes and their consequences, not in some eternal self-flagellation of continual guilt and regret, but as information which will help me make better decisions in the future. Rational hedonism is the triumph of will and reason over both false passion and social control, a perfect harmony between the self and the other, a 'golden ratio' of overall benefit and ultimate cost.
Every action I take must be a reaction to the real state of the world around me and the real actions of the people I interact with. Were it within my power, I certainly would attempt to ensure that all other beings who exist can experience the same maximal happiness and minimal suffering as myself - if only that I might thus maximize my own sympathetic happiness and minimize my own sympathetic suffering in contemplating their material conditions from the outside. All things being equal, I will happily sacrifice some happiness or endure some suffering if it will encourage enough happiness, or mitigate enough suffering, in enough other beings to be worth the degree of happiness thus delivered or suffering thus prevented to myself. Unfortunately, the cold reality is that the happiness of one often comes at the expense of another. When matters reach the point at which the benefits to myself are insufficient, I will not further forgo my own happiness or take further suffering on myself. Why would I? And I certainly may choose to risk my own death, torture, imprisonment, or violation, if I judge that the probable benefits outweigh that risk or the consquences of failure; but when the inevitable consquence of a course of action is death or something equivalent, I will of course endeavour to choose another option, even if this option will directly cause intense suffering or even death to others.
If this proposition seems cold, alien, mechanical - be assured that my moral calculus is just as emotional and off-the-cuff as anyone else's. I am not a chess-playing computer, robotically scrolling through billions of possible futures per second and choosing the most advantageous. I am human, and I suffer all the slings and arrows this mortal flesh is heir to. That is why sympathy for the other weighs so heavily in my decisions, and why I don't advocate the psychopathic 'virtue of selfishness' of a Randist, the misinterpreted and maladjusted 'will-to-power' of a Fascist, the amoral permissiveness of Relativism, or the blind pseudo-Hedonism of mechanically satiating every animal desire. But nor am I constrained by the pure-Utilitarian adjuration to 'maximize ovarall utility' at my own expense, or the moralistic lionization of noble suffering and self-sacrifice, or the absolutist promotion of material equality and the 'common good'. I take those actions, support those causes, and promulgate those ideals which I sincerely believe will ultimately most benefit me personally - both in terms of immediate material consequence, and in the sympathetic reflection of the joys and sorrows of others.