That's a philosophical question, isn't it? Our brain processes sensory input (what we see/hear/smell/taste/touch) and causes chemical reactions in other parts of the brain. Those chemical reactions evoke thoughts, and since no two brains are identical, those thoughts are not always identical. So two people can see the same thing and think differently about it.

At least that's my 2-second take on it.

We think subconsciously (regardless of will to do it) because our brains auto analyze all the information captured by all our senses, mixes it all together and creates conclusions upon your own understanding.
By own understanding I mean, who you are, what you like, what you hate, your fears, your thrives, your imagination, creativity, etc etc etc..

But in 95% of the time this is how your brains think or how the thoughts are created.

I wish I had the chemistry background as one of my best friends. Explaining to you the mechanistic processes behind vision would absolutely help you to understand where thoughts come from. I'll ask him later, but the principle is this: light enters the eye and sets off a series of chemical reactions which activates receptors in the brain which triggers particular neuro regions which control different functions (such as creating an image).

But I'm guessing you're asking what the abstraction of a thought is, or where that comes from.