I say that selfishness is impossible because selfishness always defeats itself... the self.
When we are selfish, we destroy the very fabric of the world in which we live. We destroy the 'environment,' both in terms of the physical world but also the many invisible bonds of relationship that encircle and permeate our lives.
Selfishness attacks and drains the very surroundings that, if properly cultivated, would bring forth a bounty that would even exceed our greatest desires. Selfishness cuts us off from the very things we seek and desire most.
What we must have is Respect. Respect is honoring all things in accordance with how God has made them. When we talk about self-respect, this is not Pride or Vanity, but the honest appraisal of who we are and how God has made us. We know our strengths and weaknesses. It is self-knowledge.
Knowing ourselves, we can then look to the outside world and see those around us for who they really are. We know that they are capable of and what they are not. We can see whether someone will give us what we desire, but also whether we are able to give them something in return that they value. Lop-sided relationships don't work, but become parasitic. They lack the 'symbiosis' that true relationships have.
Selfishness is not about symbiosis and harmony, but is entirely parasitic. This is why it drains and destroys.
God shows us what this symbiosis looks like by becoming one of us. Christ is incarnate and points out that He is the one in who's image we are made, and then He shows us how to give. He gives Himself, and the Father resurrects Him. We must then do likewise.
For God to do this, with no self-interest on the line, means it is a perfect gift. What we do is always 'tainted' with self-interest, because we are always beneficiaries of good. But, this is what we are designed to do. None of us is God. We all have needs.
The blessing is that God has given us a way to have self-interest, yet not the self-obsession that is selfishness. We can use our self-interest to help others. This is what service means... we get as much out of it as servants as do those whom we serve.
Selfishness cannot do this. It takes without giving. It plunders and leaves nothing to grow back. It starves itself while trying to consume the world.