Srila Gaura Govinda Maharaja:
When Krishna and Sudama were students in Sandipani Muni's asrama, they set an example. Once there was no firewood for cooking. So Sandipani Muni's wife, guru-mata, said to the boys, "There is no firewood. How can I cook? Go and fetch wood in the forest." Both Krishna and Sudama took an axe in their hand and went out. Why? This is service, guru-seva. He is the Supreme Personality of Godhead. His will is supreme. By the will of the Supreme Lord the creation and the annihilation takes place; by His mere willing. If He had wanted, immediately truckloads of firewood would have come there. What is the necessity of going to the forest with an axe in His hand on the part of the Supreme Personality of Godhead? This is guru-seva We have to do it. This fire is needed. The order came. There was no fuel, no dry wood. Therefore, they went to the forest.
Then this wonderful incident took place; black clouds appeared in the sky, a very heavy storm came blowing torrential rain. The whole land became inundated. It was very dark nightwith roaring thunder and lightining. The two boys were in the forest and could not go back to the asrama. The Supreme Personality of Godhead could not come back to the asrama! Sandipani Muni could not sleep that night - Oh, these two boys went out, and it is such a bad night with torrential rain, such a heavy storm. No one can go out of the house or even open the door. The whole land is inundated. What has happened to them? He was just thinking, and thinking in this way, and had a sleepless night.
As soon as day dawned and the rain stopped, Sandipani Muni went out to see where they were. After some time he saw Krishna and Sudama standing beneath a tree. They were wet from the rain and were shivering. They contracted such bodily pain and distress in the service of guru. Sandipani Muni embraced them, "You are sat-sisya. This body is very dear to everyone. No one wants to give distress to the body, but you both took such bodily distress and unhappiness for me. "
Krishna has said that He becomes very pleased with that person who serves guru with a loving and pure heart, offering niskapata-seva with love and affection, not caring for his own discomfort, bodily pain, unhappiness or distress.