Lord Krishna says that the appearance and the disappearance of the happiness and distress are like the appearance of the winter and the summer season.
He says that we have to perform our duty no matter what may come. We have to learn to tolerate these non permanent changes and have to continue to perform our duties.
He further explains it with an example.
He says that a person has to take his bath early in the morning even during the cold months of December and January. In the same way a person has to cook his food even in the months of May and June which are the hottest months. One has to always execute his duty inspite of the climatic inconveniences.
He further concludes that he who is not affected by either happiness or distress is eligible for liberation.