Here's some thoughts from pages 190-192 of the VimuttimaggaTHE IMMEASURABLE THOUGHT OF COMPASSION
Q. What is compassion? What is the practising of it? What are its
salient characteristic, function and manifestation? What are its benefits?
What is the procedure?
A. As parents who on seeing the suffering of their dear and only child,
compassionate it, saying, "O, how it suffers!", so one compassionates all
beings. This is compassion. One dwells undisturbed in compassion — this
is called the practising of it. The non-manifestation of non-advantage is its
salient characteristic. Happiness is its function. Harmlessness is its manifestation.
Its benefits are equal to those of loving-kindness.
What is the procedure ? The new yogin enters into a place of solitude
and sits down with mind collected and undisturbed. If he sees or hears of a
person stricken with disease, or a person affected by decay, or a person
who is full of greed, he considers thus: "That person is stricken with
suffering. How will he escape suffering?". And again, if he sees or hears
of a person of perverted mind and bound with the defilements, or a person
entering into ignorance, or one, who, having done merit in the past does not
now train himself, he considers thus: "That person is stricken with suffering;
he will fare ill. How will he escape suffering?". And again, if he sees or
hears of a person who follows demeritorious doctrines and does not follow
meritorious doctrines, or of a person who follows undesirable doctrines and
does not follow desirable doctrines, he considers thus: "That person is
stricken with suffering; he will fare ill. How will he escape suffering?".
That yogin by these means and through these activities develops the
thought of compassion for these persons and repeats it. Having by these means
and through these activities developed the thought of compassion and repeated
it, he makes his mind pliant, and capable of bearing the object. Thereafter
he gradually develops (compassion) for an indifferent person and an enemy.
The rest is as was fully taught above. Thus he fills the four directions.
Q. What is the fulfilment of compassion and what, non-fulfilment?
A. When a man fulfils compassion, he separates from harming and from
killing. He is not afflicted. He separates from impure affection. Through
two causes compassion is not fulfilled: through resentment produced within
himself and through affliction.
Q. All do not suffer. Suffering does not prevail always. Then how is
it possible to compassionate all beings ? A. As all beings have at some previous
time experienced suffering, they can grasp the sign well and practise compassion
in all places. Again, sorrow of birth and death is the common property of all
beings. Therefore all beings can in all places practise compassion.