Thought is something magical , it is as mysterious as the universe. From where it comes in , to where it goes out ; who knows. It was quite like wind which comes from some where and going to some where else. What we know is wind touches and moves. It may change the position of things, make things falling down , take valuables and go away. So what we know is the impact of wind ; not the wind. Now with weather forecasting we are able to understand the schedule of wind , its speed ,source and path. Still thoughts will fly away from the hands of modern science , who knows its agenda , schedule and impulses. Only thought itself knows it. We may think thoughts are born in brain. But who seed it , who triggers the thought. thought is as large as this universe. i don't think whole brain can hold it. If it were so brain could possibly decode all thoughts and understand the meaning of it.
Now the funny thing is thought about the thought. Thought itself is another thought ; which makes it a complicated subject. Our sky of thought is wide or infinite so that we cannot reach the boundary. It may be like two mirrors reflecting each other. It will result in infinite number of reflections.
There is a character in The Mahabharata called abhimanyu he enters into something called chakravyuha. It was like a multi tier defensive architecture created by opponents kaurava. It looks like a blooming lotus when viewed from above. It was a complicated architecture and only few people knows how to defeat it. Abhimanyu son of arjuna knows how to enter into this defensive mechanism as he listen this when he was in his mother's womb. Unfortunately he don't listen to the escape plan. His end was inside the Chakravyuha. This could be the best metaphor for thought.
We cannot take complete control over our thoughts. Just think , you break up with some one after a long period of relation and think about your ex all the time. It may happens knowingly or unknowingly. There is always stimulant for thoughts. It may be places , incidents , even objects. If you decide to forget him/her and decide to not think about ex , its not going to happen. When we start thinking to "stop thinking about something" that object/thing will be active in our thought process all the time. That's why we usually say "time heals". Its quite a natural process. Unknowingly when time moves , thought itself dump this if we don't overreact to it/try to control that thought. Thought itself know how to heal. Otherwise you will failed and ended up in sorrow.
Even if we can't control it , we could always influence our thoughts. As i mentioned earlier there is always stimulant for thoughts. Thought is influenced by ambiance. Even thought can be influenced by thoughts of others too. If you are in a church , mosque or temple you may feel bit relaxed and calm. At that point you may not thing about violence , revenge etc. It is the power of ambiance. The places we are in , the people we are communicate with will always influence our thoughts. It may be beneficial or disturbing. In the same case if you are at a hell looking place , a spot in which a murder committed you may feel very disturbed , it may haunt your thoughts. Even it may have impact on your day today activities.
Thought must be healthier. With all historical references we can say a single thought can change the world even destroy mankind. So our thoughts must bring positive energy.