Often it is because we would be so deeply moved that we would be paralyzed by our own feelings of commiseration.
During lectures when the Dalai Lama talks about compassion, he begins with "All sentient beings," and then he pauses. You can feel that he is extending his thought to many different types of beings. His voice cracks a little. Sometimes there are tears. It is as though he is saying, "Don't be afraid; don't hold back from such deep feelings."
There was a bodhisattva whose name was Always Crying because he was so concerned about the plight of sentient beings. The story is told to show how reflecting on the suffering of other beings can impact you strongly. Still, the point is to develop a better mind, and if in the process of doing so you become full of worry, that does not help other sentient beings. There is a difference between intelligent concern and worry. However, when we realize why people suffer, we can see that at least some of their pain can be eliminated, and if it can be, then it is most touching that they suffer unnecessarily. With this understanding as background, we can allow ourselves to be moved by others' pain.
In this way, realization of the emptiness of a misconceived sense of inherent existence is a cause for compassion. If you begin to understand the causes of wandering in cyclic existence, the causes of anguish, you get a sense that these can be eliminated, for it is through an error of mind that these sufferings take place. It is difficult to generate the conviction that all suffering is caused by such error, but you can see that at least some of it is.
When you see how deep the misconception of inherent existence is, when you see how pervasive its effects are, you can see how, from this one small error, huge problems are produced. It is so poignant. But because there is a way out, we can allow ourselves to open up to deep feelings of commiseration.
With wisdom and compassion working together, feelings of empathy and commiseration are built on the strength of the realization of the nature of persons and things, and analytical intelligence is warm to the feelings of both yourself and others. For the time being, these two sides have to balance each other, but in the end they are so intertwined that they are indistinguishable. Other-concern makes happy sense.