Compassion supplies the complement to loving-kindness. Whereas loving-
kindness has the characteristic of wishing for the happiness and
welfare of others, compassion has the characteristic of wishing that
others be free from suffering, a wish to be extended without limits
to all living beings. Like metta, compassion arises by entering into
the subjectivity of others, by sharing their interiority in a deep
and total way. It springs up by considering that all beings, like
ourselves, wish to be free from suffering, yet despite their wishes
continue to be harassed by pain, fear, sorrow, and other forms of
dukkha.

To develop compassion as a meditative exercise, it is most effective
to start with somebody who is actually undergoing suffering, since
this provides the natural object for compassion. One contemplates
this person's suffering, either directly or imaginatively, then
reflects that like oneself, he (she) also wants to be free from
suffering. The thought should be repeated, and contemplation
continually exercised, until a strong feeling of compassion swells up
in the heart. Then, using that feeling as a standard, one turns to
different individuals, considers how they are each exposed to
suffering, and radiates the gentle feeling of compassion out to them.
To increase the breadth and intensity of compassion it is helpful to
contemplate the various sufferings to which living beings are
susceptible. A useful guideline to this extension is provided by the
first noble truth, with its enumeration of the different aspects of
dukkha. One contemplates beings as subject to old age, then as
subject to sickness, then to death, then to sorrow, lamentation,
pain, grief, and despair, and so forth.
http://www.accesstoinsight.org/lib/bps/misc/waytoend.html
From: The Noble Eightfold Path: The Way to the End of Suffering by
Bhikkhu Bodhi, Buddhist Publication Society, http://www.bps.lk
bps@...