What factors influence thoughts the most?
As far as I see it, the issue is how to focus thoughts. Some people dwell a lot on the past. I am inclined to worry about the future too much. I do tend to think a lot about the previous day whenever I wake up. It can be so easy to dwell on the little details of one's own life. Most days I try to read straight after breakfast as this enables me to direct my focus away from my own concerns. I like to dip in and out of a few different books, usually a mixture of fiction and dare I say it, a bit of philosophy.
Your core question seems to be about what factors affect the thinking in the first place which is more about thinking disposition. Of course, some people tend to think about certain things. It is probably the case that brain development does influence this. For example, some people are born worries whereas others are optimistic. But I am inclined to think that we choose what we think about.
In particular, while brain chemistry affects people who are depressed and is likely to lead to negative thoughts it is likely that the negative thoughts give a feedback loop and give rise to further depression. So it is important what we focus upon. It is difficult though, especially if alone for long periods of time.
I think we can achieve more consciousness direction in thinking through mindfulness. I don't practice it always but I have found some techniques helpful. However, that is not about switching off from thoughts but often about direct experience of the senses and it can be about observing one's own thoughts, which is about a certain distancing from them.
So, I am saying in answer to your question while certain brain chemistry probably influences the agenda of our thought we have a fair amount of freedom to choose what we focus upon. This freedom is a positive part of life because it gives scope for us to create and maneuver inner experience rather than simply be overwhelmed by any strand of thought entering conscious experience.
I expect some people are more image based whereas others think more verbally. But all of this can be moderated by us.