Compassion fails when it causes sorrow.
When faced with suffering, if one is overwhelmed by grief or heaviness of heart, then that is not being compassionate. The quality of compassion is then tainted and not functioning properly.
When the heart is drawn towards boundlessness, it is not dragged down by suffering. Instead, it is uplifted. It is important to recognize that.
The heart could be weighed down by sorrow and grief in response to a tragic event or situation, and one could think that that is being compassionate. But that is not compassion, even though the etymology of the word (in English) is “to suffer with.” That is not the way the Buddha defined compassion. If one’s mind is affected by grief, then one is not able to respond in a clear and open-hearted manner. It is important to recognize that.
This is why sorrow and grief are characterized as the near enemy of compassion. Both responses can spring from seeing suffering in others, but grief has a depressive effect, while compassion has a positive and uplifting quality.
This reflection by Luang Por Pasanno is from the book, Broad View, Boundless Heart, (pdf) p. 18.