In a nutshell, Feinberg's argument is that "the only thing anyone is capable of desiring or pursuing ultimately (as an end in itself) is his own self-interest." Pretty straight-forward, but also a pretty heavy statement. At first, I'm inclined to think that there are plenty of things I do just because I want to be nice. For instance, I am really into random acts of kindness. I don't know how to explain this, except to say that I am known for tearing up nearly every time I see strangers do nice things for other strangers. I try to do or say at least one random nice thing for someone every day, or at least whenever I remember that this is the kind of person I aspire to be. I think that's where the message lies here. Of course, given my extreme sentimentality toward such acts, I would like to say that random acts of kindness are purely altruistic. For instance, what reason did I have to hold the door open for someone this morning? I would argue that it certainly wasn't required of me, and the person behind me was just out of the "this might actually be an awkwardly long wait to hold this door for this person" range (we've all been there).
But bear with me while I break down my thought process at the time. At first, I think there is someone behind me and so I should hold it for them because that is the nice thing to do. Then, I recognize that they're not quite within "hold the door open" range. Then I think "to hell with it, it's the nice thing to do" and so I wait. In dissecting my own seemingly altruistic action, I've already said several things that make me wonder if it's purely selfless, namely "it's the nice thing to do." I personally have a very strong desire to be nice just because I can. It isn't because I'm hoping karma catches onto my good deeds and deals me a good hand in the future; it's because it makes me feel all warm and fuzzy to do nice things for people. I'm human, after all, and sometimes I like to do nice things for strangers just as I like to do nice things for my friends and family. In part, it's because I want to impact that person's day and I would like to say that that is the main motivation. However, I think merely the fact that I said "I want to..." implies that it is, at least in part, something I do out of self-interest.
This leads me to another thought, though. I think Feinberg needs to better define what he means by "selfish" and "self-interest" because to me, these two words have different connotations. I tend to agree with his argument, but even I would like this to be defined a bit more. I'm inclined to prefer "self-interest" rather than "selfishness" because the latter has a negative connotation but I don't necessarily think it needs to be negative. Just because I'm acting somewhat in self-interest when I do nice things for strangers, doesn't mean it isn't still a nice thing to do. And furthermore, I think it's interesting to note from an evolutionary standpoint (since we're talking psychology here) that pure altruism without acting with any self-interest would really be self-defeating. From this perspective, acting in self-interest isn't just natural, it's necessary. So might be altruism at times, but not without it ultimately being out of self-interest. Sort of an afterthought, but just something interesting I thought about.