Very simply, the things which last eternally will give you eternal happiness. That which is transient, changeable, that which will die, that which will recede in time, will not give you lasting happiness.
For example, your favorite Bollywood actor movie has come and you have received the tickets. You are sitting in a very good cinema hall, enjoying your popcorn and your favorite drink, experiencing the movie which you really wanted to see for a long time. As the movie is going on and you are feeling so happy, your phone beeps and there is a message that your father is sick and has been admitted to a hospital. Now, you are in the same air-conditioned cinema hall, you have your popcorn, you have your drink and you have your favorite actor’s movie on a big screen. Yet the happiness you were experiencing just a moment ago, is gone.
You can use this analogy for everything else in this world. Anything which is born has a timeline and anything which has a timeline will go through process of growth, decay and death. This happens to people, to houses, to everything under the sky. All the things that have a date of birth, will have a date of death as well. This means that whatever you are experiencing because of these people and things, will be of a fleeting nature. Unfortunately, when we don’t use our intelligence enough, we don’t understand how things function in this world.
It is like, you have a lemon in your hand and you are squeezing it in a glass praying to God, ‘Please make it sweet, make it sweet!’ How many millions of times should you pray that the sourness of this lemon changes into a sweetness of honey?! Well, the fundamental principles do not change even if you pray. This means that what is transient will remain transient, and what is eternal will remain eternal.
And this is the beauty of Vedanta, the ultimate philosophy of Bharat. Out of the six schools of philosophy, Vedanta is the one which gives you the understanding to experience things as they are and fundamentally explains how anything functions. Beginning from your body, mind to the entire Universe. Hence, surely you will begin to understand that what comes out of this world cannot give you a lasting happiness. It can give you fleeting moments of great pleasure but it will not last.
Hence, Vedanta says that you need to search and understand. For, we all want to be happy . To experience happiness is something that all of us, not only us human beings, but also the animals, all sentient beings, are aspiring for. Lasting happiness can be experienced only when you find the source, which is everlasting and eternal, and that existence is of Truth.
Another name in Sanskrit is Brahman, or we can call it Atman. Atman doesn’t mean a gory looking form which is wearing a white cloth and is floating in the air! That is all what movie atmans are, but that is not what truth is. Atman simply means You. You in your real form, you in your reality. Once you will understand this existence which is everlasting and eternal, it will give you a roadmap to experience happiness. And this happiness will not be flickering but everlasting and eternal.