"Reminding him with a tap"

Anton Chekhov:

There ought to be behind the door of every happy, contented man some one standing with a hammer continually reminding him with a tap that there are unhappy people; that however happy he may be, life will show him her claws sooner or later, trouble will come for him -- disease, poverty, losses, and no one will see or hear, just as now he neither sees nor hears others.