That is a serious point in business these days, but to me the sign was humourous. A good sense of humour means that you don't have to be funny, you just have see what's funny. In the midst of hearty laughter we are being temporarily out of control, and it happens to have some very pleasant and beneficial effects. You do not always need a reason to laugh. In fact, if you stop to examine the reason you are laughing, you will probably kill the laughter.
Laughter erupts spontaneously from the element of surprise.
You can certainly swallow words you know will get you into trouble if you speak them. You can grind your teeth and not cry out in pain. Don't give up hope. Look for the unintended humour of reality. Then you have them available whenever you want to share with others or just to review yourself whenever your spirits need a lift.