The law of diminishing returns says, in a nutshell, the more times you do something or experience something, the less you get out of it. Usually we apply it to jokes. The first time you hear a hilarious joke, you laugh until you cry. By the twenty-ninth time you hear the same joke, you barely crack a smile (assuming you don't just roll your eyes). The return (laughter) diminished. Of course, it applies to a lot of other things, too: nerves about a new job, anger at a specific personal wrong, fear of delivering bad news, etc.
It even applies to the rage, horror, and disgust we feel at receiving bad news. Take all the terror attacks we see. September 11th brought us together as a nation. Everyone expressed solidarity with Paris and Nice. But it's just happened again in Barcelona a matter of days ago, and no one seems to feel outrage that people have been injured and killed. Too much of the same thing, including death, and the law of diminishing returns kicks in. We've simply grown numb.
If we are numb, indifferent, and callous to such senseless, needless human suffering, what can make us feel something for our fellow human beings? If we don't care that they die before their time, can we as Christians claim to care about their eternity on the other side of death? How can we say we love God if we don't love our brothers and sisters enough to mourn their murders at the hands of those who worship a false God?
As the song says, "let us not grow weary in the work of love." Let us not become cynical and numb. Let us continue to feel, to love, to care.