Full Idea
The universe is not bounded in any direction. If it were, it would necessarily have a limit somewhere, but a thing cannot have a limit unless there is something outside to limit it.
Gist of Idea
The universe must be limitless, since there could be nothing outside to limit it
Source
Lucretius (On the Nature of the Universe [c.60 BCE], I.959)
Book Reference
Lucretius: 'On the Nature of the Universe', ed/tr. Latham,Ronald [Penguin 1951], p.55
A Reaction
This is a subtler argument than the mere enquiry about why you would have to stop at the end of the universe. It still seems a nice argument, though Einstein's curvature of space seems to have thwarted it.