There's something about pain and suffering that is so universal and true. But there's also another thing about that same pain and that same suffering that makes us want to hide and retreat into ourselves. The "truth" we put out in public is not the same as the truth we see in ourselves.
Hemingway's short sentence above covers this feeling masterfully. We all come in contact with people everyday. Some of them are smiling, constantly. Some make jokes, constantly. But truly, deep down, they are not 100 percent forever, happy. It's hard to imagine what full happiness would be like. And it's hard to pretend that people don't have a story the same way you know that you have your own story and your own scars.
We need to remember that we are all going through something no matter how small or large. Hemingway lets us know how silence and being "quiet" can sometimes give off the opposite emotion and tells us that, maybe, we should be more open about our respective sufferings.