When you seek temporary things, the outcome of that is also temporary.
When you seek happiness where the root is temporary, that happiness will also last for a short time.
We often seek happiness in some worldly achievement. When you reach the summit, what do you feel ? Maybe elated for a while. Then, that same cause cannot give you new happiness.
In the same way, a careful evaluation will show that real happiness does not seek external stimuli to reach its peak, but instead continuously regenerates itself internally. The internal roots of love, care, compassion, helpfulness, forgiveness, gratitude, humility last forever. They can never disappear. They regrow by themselves giving fresh life to even a higher happiness.
When that happens, you will not feel that happiness is not lasting.