Hume here returns to a theory discussed earlier: that selfishness is at the heart of everything. Hume criticizes this theory, arguing that you'd have to have corrupt heart or use really shallow, careless reasoning to think this way.
We might assume that folks who have this opinion are unable to feel true benevolence. However, Hume recognizes that some famous philosophers maintained this selfish system but led faultless lives. Hume points out that this system can still recognize the differences among people: not everyone sees things the same way or acts the same way. Because of this, someone can be seen as moral or immoral even in this system.
Even though selfishness gets a bad rap, it's not a bad thing as long as it's combined with concern for others and usefulness to society. What Hume's doing here is making a distinction between partial selfishness and universal selfishness.
Hume observes that there's one obvious objection to the theory of selfishness; namely, that benevolence and generosity are natural instincts that are plainly different to selfishness. This is pretty obvious to anyone, so to argue that everything's based on selfishness is a stretch.
In contrast to the selfish theory, Hume points to scenarios that hinge on love for others, e.g., people's love for their kids. Hume's argument is therefore that self-interest plays a role but isn't the driving force that some other philosophers claim.