How to write about a universal truth

Many of my clients (and authors in general) write because they want to express a universal truth. Or because they want to encourage readers to accept an emotional or social concept, like self-love or the benefits of helping others. It can be tempting to state simply and absolutely: “Treat others as you want to be treated,” or “You will be happy if you just love yourself as you are.” And then just leave it at that. I advise against it, though. For one thing, that’s not very much fun, as reading experiences go. And it’s also not convincing.