I think that what is in a person's self-interest is to find a sustainable, enjoyable way to live that will last a lifetime. There is a lot more that could be said about that, but it's a high-level summary.
The proof of that is looking at man's nature and what life is. So all I can do is point in the right direction. To write it all down in a comment would be impossible, plus pointless, because it doesn't prove anything to you until you see it yourself, firsthand, by looking at reality.
> I would guess there are whole classes of people for whom stealing and manipulating is in their own self interest. Sociopaths for example.
I am certain that that's not right. Lying, cheating and manipulating will never be a sustainable mode of living. It keeps you dependent on luck and the whims of others, and eventually you will get caught. It's much better to pick any one of 10,000 honest professions and actually live by your own work. It's better to trade with "all of humanity" and live in harmony with others, than to constantly be at war with others.
> Sociopaths for example.
Being a sociopath would be horrible. (They may not realize that themselves--because they don't know any different and/or would rather not face the facts.) Why? Because you'd have to completely give up on all the values you get from enjoying other people---friendships, cooperation, and all forms of romantic love. (Sociopaths may outwardly seem like they have some of these, but in reality, everyone is not only their victim, but also a potential threat when the victim catches on to the game.) Plus the more general issues I mentioned above.
> Honest question: are you sure you aren't rejecting the parent's premise because you don't like the conclusion, not because you disagree with the premise?
I'm not sure what you are calling the premise and what you are calling the conclusion. But in general, I think that acting in your own self-interest actually results in cooperation with others and not victimizing other people. So we need to be arguing for more rational self-interest. We need to stop making the typical "moral" arguments that amount to, "If you want to be good, sacrifice for others; if you want to be happy, sacrifice other people." That is a completely horrible argument that will never result in anyone being good OR happy.