After attainment of the full Enlightenment as a Buddha, he could have found happiness without having to worry about teaching, and lived comfortably. However, for the entire period of forty-five Vassa (years), inasmuch as he had to impart his noble Teachings day and night without rest, he had to undergo a lot of physical hardship and strain. This benevolent performance with relentless effort was done simply because of his Universal Love and Great Compassion for all beings who are drifting along endlessly in the miserable whirlpool of Samsara.

All beings, no matter in whatever existence they may be, wish to escape from the sufferings of old age, sickness and death which are bound to be met as living beings. Nevertheless against their own wish, they are going through miserable conditions of life existence continuously through old, sick and eventually meeting with death. That is the reason why as a Buddha, he had taught his Dhamma knowing fully well that there was no Saviour other than him to save all living beings with his Great Compassion without regard to his discomforts and weariness. This also stands witness to the extent how he had to undergo miseries without having opportunity to gain happiness for, being deterred by his unparalleled Karuna.

This Karuna is extremely noble and gentle. It is usual for a virtuous man to have compassion and sympathy for a person who is in trouble. On the other hand, he who has no feeling of sympathy towards others in dire distress is not a man of virtue but of vice. That is why this karuna dhamma – “sevitabba, bhavittabba dhamma ” – ought to be resorted to, developed and depended upon by each and everybody who is noble. Ordinarily, even without practising bhavana, one should have a feeling of sympathy towards a pitiable person, if seen or found. Every time feeling of compassion arises, it would amount to developing merits.

If desirous of developing through bhavana, the method to be applied is similar to that of developing metta bhavana. It should not be developed beginning from those who are near and dear to one who is developing, at the initial stage. Neither should it be developed starting from the most beloved person. Nor should it be developed beginning from a neutral person or from a person who is hostile, or rather, an enemy. It is because one who is dear to you will remain as a dear and affectionate person, and hence, no compassion is called for. Similarly, the most beloved person, etc., will continue to remain as the most beloved ones and so on. As a matter of fact no feeling of compassion needs be invoked. In the case of a person of opposite sex, since lust is likely to occur, one should avoid developing karuna towards him or her at the very initial stage. Also in respect of a person who is dead and gone, no karuna should be developed as it is uncalled for. Such being the case, the question arises as to who, should be the first towards whom karuna is to be developed.

Source: http://www.buddhanet.net