So I Wonder....Where DO They Come From, and Why?

Thoughts.
Where do they come from?
Why?
Now I know this has happened to everyone before, where a random thought pops into your head and you wonder what made me think about that. I will give an example:
I am walking around Soho and I am about to cross one of the busier streets and see cars coming, but at the same time I see a rather chubby lady maybe 2 feet in the street attempting to cross. So immediately after seeing this lady this pops into my head.
"If this fat bitch can make it across the street, I can"
Now can someone tell me where that came from!
I am not an openly rude person, while I'm not known for voicing my first opinions on things frequently but my initial reaction to the thought was another thought "where did that thought come from?"
Then I began thinking, why do we think the way we do and, what percentage of our thoughts do we actually control? And I have been convinced that we do not %100 control of our thoughts by this simple incident.
Now with that said can someone tell me where our thoughts come from?