Every man’s reason for living, regardless of personal philosophy or religion, is to satisfy himself to the best of his abilities. Every decision he makes is for his own benefit (or perceived benefit). There are no exceptions.
The natural conclusion that follows is that true selflessness does not exist. This conclusion is true. No decision is motivated by anything greater than the motivation to maintain high personal satisfaction. For the sake of clarity in the OP I will provide counter examples.
1. Philanthropists – Generous people who risk life and limb to help those in need are not being selfless, per se. They are acting for themselves by helping others. Essentially, they are maintaining personal satisfaction by maintaining others. If they did not help others, they would not be satisfied. If they didn’t “feel good” about helping others, they would not do it.
2. Religious people – Religious people who sacrifice their belongings and time, and comply with laws that go against their biological nature are completely motivated by their own interests. They are not “doing things for god”. They are doing things so that they go to heaven. If God said “Regardless of how much you give, you will go to hell.”, there would not be a single man who resisted his natural urges, instincts, and desires.
3. People in love – In a situation where a mother would sacrifice her life to save her child, the mother is not necessarily being selfless. At this point I would like to introduce something I have labeled “The happiness threshold”. It could very well be called the suicide threshold, but I am an optimist. Any level of personal satisfaction above the threshold represents a desire to live and experience perception. Anything below the threshold represents a lack of desire to live; it is a desire to die. A mother who lets herself die to save her child is gambling that if she were to let her child die, she would dip below the happiness threshold and experience overall dissatisfaction. She is gambling that life would not be worth living. She is making a decision based on her own desires and satisfaction.
Feel free to offer any counter examples to my assertion that nobody is selfless.
TL;DR No man cares about another more than he cares about himself.