The reason is the fact that your happiness depends on something outside such as environments, circumstances, and conditions. To go beyond happiness and to feel good all the time, you need to focus on something within, something inside. Because outside conditions necessarily can be controlled by you all the time.
How can we achieve bliss? The answer is understanding. Understand that you are not your mind. Understand that existence is absolute. Understand that the world you now think is just an illusion made by your mind.