It is true that people are egoistic whenever they are acting in accordance with reason. What psychological egoism overlooks however is that people have the potential to act against their reason. Altruism occurs when reason loses its control and emotional or involuntary actions occur. In order to qualify as truly altruistic, the action must be done for the sake of others and solely for the sake of others, so even reasons such as "in order to be ethical" do not count as altruistic in nature, because it is in order that the individual may receive the benefit of being ethical.
Humans do all sorts of things to help or benefit others, but they are actions that will also benefit the person doing it in some way, whether it be some physical gain, or something as simple as recognition, so it would not be a case of "true" altruism. But many times people do things in situations where they do not even have time to think, they will just do something to save another's life or whatever, and if they have not had time to think in advance they they are not doing it for any benefit of their own.
Yes, it's possible for humans to be altruistic, which is to say that they will be more concerned about others' interests than their own. We see this on the news sometimes when someone puts himself in harm's way in order to save a stranger, though, of course, it happens every day in smaller ways. In a world where people are becoming more and more selfish, it's the stories of altruism that truly stand out and make us keep our faith in human nature.
Let's suppose you have a choice between buying a life insurance policy that will take care of your family when you die and taking a pill that will give you the exact same psychological pay-off as getting the life-insurance policy. The only difference is that you will have to pay for the life insurance policy, but the pill is free. The only possible self-interested motive you could have for getting the life insurance policy is psychological since it doesn't pay out until you're dead, and you don't get any money out of it. If you could get that exact same psychological pay off from a pill that doesn't cost you a dime, and if you really are acting in self-interest, then you would choose the pill. Yet anybody who loves their family will choose the life-insurance because their real motive is not to get the psychological feeling of comfort and security that comes from knowing their family will be taken care of. Rather, the real motive is simply their love and concern for their family. Psychological egoists make the mistake of confusing the result of an action with the motive for the action. The fact that acting in the interest of others has a benefit to the self doesn't mean the benefit to the self is what motivated the action. A moment's inward reflection will reveal that we are all frequently motivated by the interests of others, and the fact that we are emotionally invested in the interest of others does not make our actions self-interested, nor undermine the fact that they are other-interested.
if they don't think, then they aren't actually choosing another's well-being over their own. It may be "truly" altruistic, but I would add that a "truly" altruistic act would not be recognized, as such, by the actor. It would simply be like putting one foot in front of another. Faced with something that just needed "doing", any pause or hesitation would automatically strip it of "altruism", for the reason Ethan gives.
It just isn't our nature. No species are altruistic, as any natural creature judges based self gain. The modern social depiction of what's "right" is to sacrifice one's self. Think of this on an evolutionary scale: If everyone was selfish, would there be a need for altruism? Please excuse my rhetorics.
Everyone is self-interested in everything they do by definition. Even haightstreet's person is acting on one's feelings and one's feelings reflect an attitude which is self-interested. Happiness derived from caring about other people is a form of self-interest as it is an interest of the self. But it is because human self-interest often coincides with the welfare of other humans that the human race has been so successful, and the positive feelings I felt as I thought of that, that feeling of a connection to humanity and part of something greater, yes it is a self-interest but it is the most strong and important one we have.