It's fairly easy to see the geographical barriers that separate the other former parts of the Roman Empire into separate countries(Pyrenees, Alps, the Mediterranean) but as far as I can tell there is not an exaggerated boundary between Portugal and Spain. I even remember reading somewhere that all of Iberia was joined in a union at one point, however it obviously didn't last. Why would the people of Iberia not have united of their own accord beforehand?
You can see answers to all of these question in the subreddt faq.
Questions/threads that have been answered there include:
Why do Spain and Portugal have different languages?
Border Between Portugal and Spain
To what extent is Portugal just another one of the Spanish kingdoms?
How did Portugal remain independant from Spain?