Happiness is the greatest cause of unhappiness.
The obsessive search for happiness, the addictive idea that we should be happy and if this is not the case then our life is a failure is what primarily causes our pain and misery.
The reality is that our happiness is regularly frustrated and is always bound to end at a certain time
We may be happy in a certain moment and unhappy in another.
Moreover, someone may be happy now, whereas somebody else may be unhappy. In this way happiness also becomes competitive, a source of conflict between those who are happy and those who are unhappy.
Happiness is the greatest cause of unhappiness, when you believe that you should be happy, that life is beautiful, and there is something wrong if you are not happy or your life is miserable.
The pragmatic reality is that happiness and unhappiness are part of life, that life can be beautiful as well as miserable.
When you fully realise this then all that counts in your life involves accepting both happiness and unhappiness, joy and misery.
Acceptance becomes your guiding principle, what you learn, which also includes accepting that you cannot accept.