Making a joke every time someone gets serious with you is a pretty good way to never hear the truth.

A joke at the serious moment works, which is the problem. It breaks the tension, everyone laughs, and the thing that was about to be said gets put away. Do that often enough and people learn not to bother starting, so you end up protected from exactly the conversations that would have told you something. The protection is real and so is the cost.

Notice the next time someone shifts into a lower, slower voice with you, and let the silence sit instead of filling it. Say one plain sentence back, even if it is only that you are listening. You can be funny again once they have finished.