Spain only formed because the Kingdoms of *Catille and Aragon united in a marriage (there were more kingdoms before that, Asturias, Galicia, Navarra, but there's no need to go that far).

Portugal was about as different from the Castillian people as the Catalans (Aragon's people) were but they were never inherited into the kingdom and remained separate.

An interesting fact about this whole thing is that the Spanish language was actually very diverse with many different variations in it (I'm not sure how many survived).

Portuguese back then was as different from Spanish as any other branch of it so, had it been inherited, it could have become part of Spain and the Spanish language.

However, it has developed too long as a separate nation to be considered similar enough now. I believe that if Aragon was never inherited, it would be the same case for them (and we would probably have had an interesting Catalan-Italian culture dominating the Western Mediterranean due to Aragon's lands in Italy and the possibility that they wanted to take more. Or maybe it would've been like Austria-Hungary with one country ruling over several different cultures but this would belong in /r/historicalwhatif).

Edit: *Castille

There's a few good discussions about this topic already.

Check it out: http://www.reddit.com/r/AskHistorians/comments/149fgx/to_what_extent_is_portugal_just_another_one_of/ http://www.reddit.com/r/AskHistorians/comments/17hfbv/how_did_portugal_remain_independant_from_spain/