Aah, Why thoughts? Why should there be constant stream of thoughts?
If one looks at it, one would find the reason of existence of thoughts simply is that the one enjoys to have thoughts! That's it. Nothing more!
You subconsciously enjoy thoughts, for that's all you have experienced. You never experienced food, beauty, river, tree, etc. but the thought that chipped in during direct experience. So, when thoughts all that you have experienced & enjoyed, then you tend to bring about thoughts subconsciously to enjoy them.
BTW, the thinker is also thought. :)