5.21, Sloka 21, Chapter 5:
This sloka is the essence of Gita and a very powerful one. Those Who are not attached to external sense objects realize divine bliss with the self.
A person who is not happy with himself cannot make the world a better place. Why shouldn’t we be happy with our own self? If a person realizes the ultimate truth/ultimate meaning of life and starts doing things accordingly he can be happy. If a person is happy he can make the world a better place.
Lord Krishna says such people who seek ultimate truth/meaning of life see unending happiness though uniting meaning of life with self.
5.22, Sloka 22, Chapter 5:
To support the earlier sloka Lord Krishna gives another Jewel of wisdom. He says all things which are external, which people seek usually have a starting and an end. They give pleasure in the beginning and they give pain once they end. There is no sense object in the world which can give happiness forever. All external things are transitory. So, if a person rests his happiness on those objects, then he will derive pleasure for some days and after some time, the pleasure ends. It has been the quest of mankind to find unending happiness. The answer is within the man himself and not on any external object.
Lord Krishna also says that the sense objects though seem to give pleasure, in the beginning, they ultimately cause misery. For unending happiness, we have to look within a person and find answers as to what causes ultimate happiness and pursue those activities that will lead to happiness. We usually confuse happiness as something that makes a person laugh all the time, be jovial, behave like an extrovert etc. Those may be some symptoms of happiness. I feel lack of sorrow is happiness. If we are not feeling sad that state is called happiness and it can be achieved by leading an active life completely absorbed in ultimate meaning of life.