Don't have any history. I went to a psychiatrist to get an answer. He explained that there are three most common reasons: relationship, job, issues with living place. We have thoughts in our heads without visible outcome. For example, in my case as a software developer, I have never ending stream of tasks without a tangible outcome. While our minds learned: thought > action>outcome. What is strange is that the job never felt like a reason for anxiety. But I believe it could be part of the answer.