One of the simplest analogies that Hume uses in the Enquiry is the building of a wall. Explaining how society works (or should work), Hume makes this comparison:
The happiness and prosperity of mankind, arising from the social virtue of benevolence and its subdivisions, may be compared to a wall, built by many hands, which still rises by each stone that is heaped upon it, and receives increase proportional to the diligence and care of each workman. (AIII.5)
Though he recognizes that self-interest isn't a bad thing, Hume argues that it should be combined with usefulness and benevolence—not just toward the people closest to us but to wider society. It's a kind of social contract where everyone does their part and everyone benefits. If only a few people worked on building a wall, then it'd be a lot slower and much harder work than if lots of people helped out. Teamwork!
You've probably heard this analogy before, so it's not like it's some big revelation. Still, it does an awesome job of summing up Hume's argument.