As a member of a society, you have obligations. You have to pay taxes. You should really try to save people if they're drowning near you (unless you can't swim or don't want to get your hair wet). And finally, if someone utters the number 69, you must acknowledge it. How you do so is up to you -- a simple "nice" is preferred, but in more formal settings, perhaps a rakish grin?
For an illustration of why this fundamental act of human decency is important, view the clip above. It's late night at a McDonald's in Manchester. The scene is stressful. The crowd, you can tell, is hungry. And the workers are dealing with a likely inebriated mob. It's tense.
One worker stands behind the counter shouting order numbers as customers reach toward him and vie for his attention. As the camera zooms in for a close up on his face, he bellows, "Order 69." Suddenly, a pride of young men cheers, and the camera pans to them. The rest of the restaurant joins in. The woman filming lapses into giggles.
And just like that, the tension is diffused. A sense of community develops. This is what civilization is all about.
It's the least you could do.