He was the ultimate "do it for somethings own sake" type of guy. He had a ton of curiosity, and just spent most of his life indulging it to the fullest. I mean, we in the modern world are so boxed in with our notions of right and wrong (most of which come from "authority"), newton wasn't like that, the man was equally comfortable studying about the bible, alchemy, as we was studying the natural world, building telescopes and as you said polishing his own lenses.
Newton was the pinnacle of this kind of personality, but frankly this was the common mindset until the 19th century. Science was not a profession, people who did it were on their own dime and their work was basically an expensive hobby. Things change a lot when you have to "publish or perish".
