Every act of a man has certain results, which he cannot anticipate. In the process of doing something, we might incur the enmity of many. Those enemies, in turn, may harm us. We do not anticipate these, and when we have to face them, we are filled with fear and sorrow. We see this in the case of Drona and Drupada, said Kidambi Narayanan in a discourse.
Drona and Drupada were friends in their gurukula. Drupada promised Drona that once he became king, he would give half his kingdom to Drona. But when he became the king, Drupada forgot his promise to Drona. The latter too did not insist that Drupada keep his promise. But Drona was poor, and his son Asvattama was mocked by his friends on account of his poverty. So, Drona approached Drupada and reminded him of his promise. Drupada insulted him and said he could not be the friend of a poor man like Drona. Drona vowed to avenge the insult. He eventually deprived Drupada of half his kingdom, and Drupada performed a sacrifice to avenge himself on Drona! Out of the sacrifice came Drishtadyumna, born for the purpose of killing Drona. So, a broken promise had a chain of consequences.
It led to a fight, with the defeated king losing land to the victor, which led to a yaga, from which emerged a killer. Thus, once the initial act was performed, none of the results could be contained or controlled.