"Why do we experience Pain or Pleasure?" Once we know the cause we can take action to reduce pain or prolong pleasure. Law of Karma basically tells us Pain & pleasure are on account of our Prarabdha karma phalas—the fruits of past actions. There are three kinds of Karmas—Sanchit karma or Accumulated karmas the sum of all fruits, good and bad obtained in the past births. E sometimes see a person who does not care for others, yet leads a good life. This is because of the good past karmas. But if he is troubling others in this life he is sowing bad seeds for future birth. This is how karma works.
Prarabdha Karma phalas are a part of sanchit karma, allotted to us for this birth. These are unavoidable, as a shooter cannot retrieve back the arrow which has left his bow. Hence they are exhaustible only by experiencing the same.
Agami karmas are the deeds performed in this birth that would affect the rewards in future births; as these karmas get accumulated with Sanchit karmas.
Once Yamraj was visiting Vishnu at Vaikunth, at the entrance he finds Garuda, Vishnu’s vehicle and he also sees a small sparrow by Garuda’s side. Yamaraj is surprised to see the sparrow there & walks inside. Sparrow seeing Yamaraj there thinks it’s time is up & starts trembling. Garuda assures sparrow to move her to safe place in no time as he can fly at the highest speed of wind. He takes it to far-off Gandhamadana Mountain near Rameshwaram. He tells sparrow that it would be safe there and gets back to Vaikuntha.
After some time when Yamaraj comes out and finds sparrow missing, he asks Garuda about the same, who tells him that it is now far away and safe. Yamaraj asks Garuda where he took him. He replies that he took the sparrow to Gandhmadana Mountain. Hearing this Yamaraj is amazed at the intricate ways the Destiny works and tells Garuda that when he came he was surprised to see the sparrow there since it was supposed be killed by an eagle in few minutes at Gandhmadana Mountain and wondered how the sparrow will fly so far in such a short time.
Such are the mysterious ways of Karma, finding ways to make one realise the fruit of action –past or present. Hence one must build up the Bank balance of “Good” karmas as defined in Geeta.