That there is something externally you have to do in order to experience happiness.
You say to yourself that once you are done with your education, you can be happy – no more exams, hurrah! Or once you have found the love of your life, got the job of your dreams, the perfect body or a big house at the beach, then you can allow yourself to be happy.
In other words – you attach happiness to an external event.
The reason may not be obvious but every time you reach a goal, new car or a new and better job, you merely move the goalpost of what success looks like, making it impossible to reach happiness, hunting for external goals.
In the bigger scheme of things, happiness and sadness are mere distortions of mind and has nothing to do with external events that you think are important to make or keep you happy.
There is no prize in the end and nobody has figured it out completely. Everyone is struggling, just at varied scales. It is not about your achievements, neither the imaginary staircase you keep climbing everyday.
In reality, happiness is all about the journey and not specific destinations.
There is no path to happiness, Happiness is the path. ~ Buddha