Why should one develop compassion?

In the Mahayana, the standard reason is that compassion is a step on the way to Bodhicitta, which is itself a necessary precursor, as motivation, to attaining full enlightenment, the state of a Buddha, the goal of the Mahayana and state of maximum benefit to sentient beings.

Unpacking that a little...

To develop Bodhicitta, one first meditates either on the kindness of others or on their essential similarity to oneself in wanting happiness, or both. With those realizations, one next nurtures compassion for others, and ultimately great compassion wherein you simply cannot bear to see anyone suffering. In that state, you not only wish others to be free of suffering, you resolve to free them yourself. Combining that with your understanding of the Four Truths, you realize that freeing others from suffering really requires that they attain liberation, and that the best way for you to help them do so is for you yourself to gain the unlimited insight and skill of a Buddha.

Hence, your motivation to attain Buddhahood is driven by great compassion and gains the intensity needed to achieve the goal. This is what is known as Bodhicitta and one who attains is it called a Bodhisattva, a being fully devoted to the welfare of others and on the way to Buddhahood.

Of course, this is a schematic account of the process -- in practice you spiral among levels of these stages, increasing your compassion, motivation, insight and skill all the while until attaining the goal.

As you can see, compassion is thus intimately intertwined with this process, as both cause and effect. And at the goal, you simultaneously attain both perfect compassion and perfect wisdom together and see them to be simply two sides of the same coin.

That all sounds a bit exalted and beyond everyday experience, though the manifest compassion of accomplished masters is evidence that it is possible. But I like to think of it this more visceral way...

I am aiming ultimately to deeply, transformationally realize emptiness or equivalently dependent arising. Having and acting from compassion behaviorally strengthens my grasp of interdependence and undermines my false sense of independence, or as it's called, inherent existence. Or even more succintly, compassion undermines the sense of an inherently existent self set against others and this promotes my grasp of emptiness.

The central belief in Buddhism is that we are not who we naively think we are. The central question is, well, who are we then?

In the Mahayana version, a hard to follow discussion leads to the us that nothing is causally independent and as a consequence, we should reframe the question of liberation (from what ever problem brought us to seek the answers that religion provides) to how do we solve this problem for everyone. No one is enlightened until we all are enlightened.

In the various paths of liberation, it the Mahayana version, re-orienting the goal to include everyone (dealing with everyone's problems, not just your own), is the first (or first of two) steps:

http://en.wikipedia.org/wiki/Buddhist_Paths_to_liberation#Lam_Rim

So to raise the motivation to reach liberation via the liberation of all, it helps to dwell on the misery of others & to follow through to act on that.

The Buddha showed compassion: that's stated in the Ayacana Sutta,

Then the Blessed One, having understood Brahma's invitation, out of compassion for beings, surveyed the world with the eye of an Awakened One. As he did so, he saw beings with etc.

"Not hating" may be enough: the fifth verse in the Dhammapada says "non-hatred" not "compassion",

Hatred is never appeased by hatred in this world. By non-hatred alone is hatred appeased. This is a law eternal.

There is an article Arahants, Bodhisattvas, and Buddhas by Ven. Bhikkhu Bodhi which mentions different views of compassion within different schools of Buddhism.

Its last section is titled "Breaking down old stereotypes" and says that there are two negative stereotypes:

(1) Arahants, and Theravādin Buddhists, are concerned exclusively with their own salvation as opposed to the benefit of others; they have a narrow fixation on personal liberation because they are "fearful of birth and death" and therefore have little compassion for others and don't undertake activities intended to benefit them.

(2) Followers of the bodhisattva ideal, and Mahāyāna Buddhists, are so much involved in social projects aimed at benefiting others that they don't take up the practice that the Buddha assigned to his disciples, namely, the taming of the mind and the development of insight. They have overwhelmed themselves with social duties and forsaken meditation practice.

Saying that the first of these stereotypes is untrue:

The example established by the Buddha's great arahant disciples has been the model for the followers of the arahant ideal throughout history. While those who pursue this ideal do not make such lofty vows as do followers of the bodhisattva ideal, they are inspired by the example of the Buddha and his great disciples to work for the spiritual and moral uplift of others to the best of their ability: by teaching, by example, and by direct spiritual influence, inspired by the Buddha's command to "wander forth for the welfare of the multitude, for the happiness of the multitude, out of compassion for the world, for the good, welfare, and happiness of devas and human beings."

It says that "giving" is important:

Those who seek the goal of nirvāṇa do not wait until they become arahants before they start helping others. Within this system, giving is regarded as the foundation for all other virtues; it is the first basis of merit and the first of the ten pāramis.

For a beginner, I guess that compassion might help:

• Harmlessness (if you feel compassion then you don't want to cause harm)
• Equanimity (if someone annoys you then feeling compassion for them is an antidote)
• Non-attachment (compassion could cause generosity, practising generosity might help to develop renunciation, patience, and other virtues)

One reason is that compassion softens self-boundaries the most.

Hatred strengthens boundaries of the self. Someone does something contrary to what we value, and our emotional reaction not only strengthens our attachment to this value, but our attachment to ourselves, as we identify more strongly with us vs. the other (exemplified by that person).

Not hating (letting go) softens this for obvious reasons.

Compassion goes further, for to positively work towards the benefits of others (especially those we would normally hate) is to further soften our self-boundaries, by putting our time and energy into their welfare, and in a way, to start thinking of their welfare as our welfare (in a way, this is what caring is).

I think the best explanation for compassion's role in softening the boundaries of the self doesn't come from Buddhism but from... Einstein. Here's a very nice quote of his:

A human being is a part of the whole, called by us "Universe," a part limited in time and space. He experiences himself, his thoughts and feelings as something separated from the rest, a kind of optical delusion of his consciousness. This delusion is a kind of prison for us, restricting us to our personal desires and to affection for a few persons nearest to us. Our task must be to free ourselves from this prison by widening our circle of compassion to embrace all living creatures and the whole of nature in its beauty. Nobody is able to achieve this completely, but the striving for such achievement is in itself a part of the liberation and a foundation for inner security.

Now you may not agree on how completely we can achieve this, but the central idea here is that compassion relaxes the boundaries of the self, expands our sense of one-ness, and to the extent we can express compassion, we are liberated.