Our awareness of feelings in the body and mind ranges from simple frustration and malaise to anguish, despair, and white-hot physical pain, and from simple pleasures to extraordinary ecstasy. As we become clearly cognizant of the bandwidth of our own feelings, we direct our awareness externally. We become vividly aware that myriad sentient beings around us are not simply objects of our pleasure, displeasure, or indifference, but have feelings just like ours. By turning our awareness outward and closely applying mindfulness to other sentient beings, we can empathize with their feelings. When we empathize with another’s suffering and we attend closely, compassion arises. The suffering of unpleasant feelings is the very source of the experience of compassion.
From Minding Closely: The Four Applications of Mindfulness by B. Alan Wallace, page 146