Exactly as the title says. Tell me why being selfish is wrong.
I'm not of the opinion that a rational, considered self-interest is wrong. I think it's usually the best way to go. In fact, I would say it's kind of like our default setting, but I've seen quite a few people post recently about how being selfless is good. I disagree.
Why do you think that selflessness is good? Why do you think that selfishness is bad? Do you think it's the other way around? Or maybe neither is good nor bad?
"The first being that 6 strangers do not magically stumble across some random cake."
Such was never stated, and such complications and variables were acknowledged in the post. This is not a disagreement with your considerations, but the purpose of the hypothetical scenario was to illustrate basic principle. The fact that it doesn't represent all variables may make it seem unrealistic; however, this is not entirely true. In real life, our perspective is too limited to appreciate every variable so we have to act without taking everything into consideration. This is where principles are especially valuable. Sure, given different context and variables, my principles will manifest into different actions, but the root of those actions still remains fairly constant.
"Even the definitions that you have given equate self-interest with selfishness."
They don't equate, but they do overlap. Elements exist in one definition which do not exist in the other. My position is not that your usage is incorrect, but rather that the distinction is worth preserving in the context of this thread. In my opinion, the type of selfishness to which people typically object resides in that distinction.
"My position is not that your usage is incorrect, but rather that the distinction is worth preserving in the context of this thread."
What I've gotten out of this thread overall is there are no real substantive criticisms of Sagacious Hawk's position. We got some snark directed at Ayn Rand, a few people who said "duh!" and people who pretty much agreed once they understood the concept of rational selfishness (though many want to call it something else).
The initial post didn't seem like it was really framed to garner criticisms, but rather to get people's general views. The position put forward in the OP was quite general, and with a qualifier like 'rational', it is difficult to take issue. It seems to imply that needlessly destructive behaviour is excluded. Rational, considered selflessness would also be difficult to disagree with. What I don't object to is being rational, and I accept that self-interested, selfish, or selfless actions can all be fueled by rationality.
I disagree that rational, considered self-interest is 'usually the best way to go' if it is a statement of principle rather than an observed trend. In most cases in my life, my considerations are a balance between self-interest and group-interest and there isn't a need to make it strictly one or the other. While group-interest may be a product of self-interest, the two sets of considerations are ultimately different. What benefits the group will not always benefit me individually and what benefits me individually will not always benefit the group. These two things are definitionally distinct.
More than focusing on self-interest or group-interest as generally preferable or better, I think it is simpler just to try to act in a balanced manner as context dictates so that the averaged out impact of my actions puts neither me nor the group substantially ahead. Sometimes I will commit selfish acts to achieve this goal. Sometimes I will commit selfless acts to achieve this goal. Often my acts will be quite neutral. Where I may align more closely with the original post is if the balance between individual interests and those of the group cannot be perfectly balanced, I think it is better for individuals to place their own interests slightly higher.
When I use the term 'the group' this is merely a recognition that I do not exist in isolation and that contemporary humans are highly interdependent. There are many cases where my interests don't significantly intersect the interests of others, in which case this conversation is not relevant.