From studying the brain scientists have found that repeated thoughts actually create physical grooves in the brain. When we practice a skill, learn a sport or study facts, a little trench is carved into our brain tissue. This is another reason why it is hard to break a habit. A habit is truly physical. One must make a new brain groove to break a habit.
Repeated thoughts become not just brain grooves, but deeds and repeated deeds become concrete routines. This natural functioning condition of the brain helps us to learn in the best-case scenario. In a worst-case scenario these thoughts become ditches of self-doubt, phobias, obsessive thoughts, vices, misdeeds and worse.