To me it sounds more like standing in wonder at how vastly much more he could never have lived long enough to understand. It doesn't strike me as sad, but humble, and appropriately so.
Newton may have been rough/arrogant with other people, but this is entirely consistent with being humble in the face of the mysteries of the universe etc — there is nothing in his biography to suggest otherwise.
In fact, you can see hints of it even in this quote: “I don’t know what I may seem to the world” — suggests that he thinks of himself in an exalted position wrt the world (= other people, in this context), but a little boy wrt Nature. Something like: I am a little boy collecting pebbles on the shore, while everyone else is a senseless baby not even close to the ocean.
