Compassion is defined as a feeling of sympathy or sorrow for the misfortune of others.
You can feel compassion because one way or another you have felt what they have felt in their trying times. Even though you don't really know the extent of their pain or suffering, you can imagine what they are going through and you feel for them.
You show compassion to grieving individuals by simply being there for them. You listen to their problems without judgement. You show compassion by being kind to other people and for treating those who are less fortunate with utmost respect and not stripping them of their dignity as a human being.