Psychological Egoism does not make sense because everyone does not always act in their own self-interest. The defenders of Psychological Egoism do not give us compelling reason to think that no one ever chooses to do something that is not in his own best interest. It is impossible to prove Psychological Egoism due to the principle of falsifiabilty.
Psychological Egoists think human nature is completely and absolutely egoistic. They have the idea that all of our actions are conscious decisions we make ourselves. People are entirely selfish and if we choose to help someone, we do it for our own benefit. Their conclusion is that even unselfish acts make us feel good so we are truly acting in self-interest. This argument assumes we are all the same and people never intentionally do anything except what we want to do. This doesn't mean we are all selfish in the way we understand selfishness.
Psychological Egoism confines human ambition to a single cause, but we are all motivated to do what we are motivated to do. Attempting to define each and every reason by each and every person for each and every act, consciously and/or subconsciously, is impossible and can't be reduced to one cause. The desire to preserve and protect one's own life may be basic to most people, but that statement can't be proven.
The problem with Psychological Egoism is that those in favor of the theory don't allow for any possible action to count as evidence against their theory. The theory changes from a theoretical description of human nature to an assumption about human nature. If Psychological Egoism is only an assumption about human nature then it is just as credible as Altruism (people sometimes do sacrifice their own interests for the interests of others). But if Psychological Egoism is true then Altruism can't be true. An assumption can't be supported, which makes Psychological Egoism unfalsifiable.