As a psychologist who has worked with individuals, couples, and families for over twenty years, and being a fellow human being with my own vulnerabilities, I have learned that there are two causes of misery:
1) Overly wanting what you don't have, and,
2) Overly NOT wanting what you already have.
I really believe that it comes down to those two reasons why we become miserable.
Don't get me wrong, though. Certainly our healthy striving toward the goals we desire can be a wonderful stepping stone on the road to mental health. But OVERLY wanting what we don't have puts us on the misery expressway.
Alternatively, not wanting some things or situations that we have does not have to be so bad. For example, not wanting to stay in the same apartment or job for twenty years is certainly reasonable. But OVERLY NOT wanting what you have in your life is sure to create despair.
We will be far less miserable by being mindful of just how attached we become to what we want and don't want. In youth we are driven to amass as much as we can, as fast as we can. In contrast, we see the elderly who tend to be sitting and observing, seemingly content to still be alive and just take life all in.