We're often pretty careless about the notion of interests. We apply it without much thought to cases that appear obvious. We say things -- not necessarily all at the same time -- that suppose that happiness, satisfying someone's desires, getting more money and living longer are each in a person's interests. But what is it that unifies these things? What is it for something to be in your interests?
For example, is something in your interests if and only if it satisfies your desires? Doesn't that depend on what kind of desires you have? If you desire the welfare of others, then a benefit to them might satisfy your desires, but it's awkward to say that, for that reason, a benefit to them is in your interests.
Try again. Suppose something extends your life. Does that make it (other things equal) in your interests? Well, it depends on whether life-extension (other things equal) is in your interests.
In fact, almost anything you say can quickly be shown to give counter-intuitive results or else to be circular. The counter-intuitive results show up when one tries to press some particular definition of interests, such as contribution to longevity. Most people agree that longer life is not always in a person's interests. It can be worse for you when, for example, it is accompanied by great suffering. But if you avoid counter-examples, you run into circularity: If something is in your interests, then it is a benefit to you. What's a benefit? Something that's good for you. What's something that's good for you? Something that's in your interests.
I think a more realistic picture is not to start by thinking about "self-interested vs. non-self-interested."? (We can try to bring that in later.) Instead, what people do start with is a set of nature-given desires or drives to do or have certain things that we find to be satisfying for their own sake -- i.e., not just as means to something else. In the course of growing up and education we can be trained to desire other things for their own sakes. As we mature, we can even become collaborators in this process of training ourselves to want or seek certain things for their own sakes.
If I'm right, then we don't start with various pre-labeled drives or desires that are self-interested. Instead, we start with a bunch of different things that we want. Sorting these into categories of "self-interested"? and "not self-interested"? is a later achievement. I don't know of anybody who has done a really good job of analyzing and teasing out all the implications of the idea of what is in one's interests. (If you do, please let me know!) That doesn't mean that we have no idea what we're talking about or that we can't make pretty reliable assignments a lot of the time -- but as far as I can tell, it's pretty rough and ready and there are plenty of apparent border-line cases that don't seem to fit comfortably on either side.