It certainly sounds like KPO (Keen Perception of the Obvious) to say that we should all live for own self-interest, but the fact is many of us don't, and that many people try to convince us not to and clearly the most difficult aspect of this self-interested approach to existence is having an uncompromised recognition of what our self-interest is exactly. Why? Because so much of what we do is in conflict and self-interest can easily become self-indulgence and lead you down a path to misery or another Stephen King novel.
My pleasure is often in conflict with my health. Responsible behavior is constantly getting in conflict with the time I have available. I acquiesce to the wants of others even though I sense trouble because I believe it is in my self-interest to play nice. I don't play nice and pay the price.
Adam Smith decided that in economics you should have none of these dilemmas that complicate other areas of your life. Having more capital is always deemed to be superior to your self-interest than having less capital. I have pondered this, trying to find a hole in his reasoning, but I see none. All other things being equal, I can find no circumstance in which more capital would be a hindrance except if you were in the company of bad people and more capital would make you more of a target for their evil schemes. And perhaps there is the major drawback of having more capital, evil schemes, but let's ignore that.
Let's just assume that having more capital will allow you to do more things for yourself, which has more potential to benefit your self-interest than having less, unless like me you are a person who habitually makes bad decisions with your capital which means having more capital would only allow you to make more bad decisions, or even worse decisions, but if you had enough capital you could hire a capital expert to make those decisions for you, but you'd first have to find a way to keep more of the capital you get to be able to afford the capital expert, which might require you to change your habits and thus not need the expert after all. But I digress.
In conclusion, in many things it can be said that pursuing self-interest is not guaranteed to improve your lot in life, but I think I've clearly shown that in terms of capital, all other things being equal, it certainly should. If not in reality, then perhaps it could have a placebo effect.
Based on the work of Adam Smith, not closely related by blood that I know of.