The strongest of personalities may have the weakest of moments. You will never know when you catch someone at their weak moment. At that moment, the smallest of things said, can scar someone for weeks, months or even years.
Each person is fighting a battle within. Some at all times, some at certain times. We have no idea what that battle is all about, we don't know where it started and what can aggravate it. We don't know how what we say can impact it.
The wisest thing then, is to choose kindness. Never belittle anyone for the smallest of reasons, or the silliest of jokes.
We may never be able to help someone out of their inner struggles. Let us consciously try not to make it worse.