The only thing that I can think of to account for this is that I grew up with my first few years listening to Japanese every day (and possibly, too, that I was living in Japan from 3rd through 9th grade), whereas I learned Spanish after the age of 30. (Maybe, too, there is some additional part of this that has to deal with the fact that - perhaps due to the myriad homonymous words between Japanese and Spanish - there is some level of internal discord between the languages in my mind.)
Would I have liked to have had Japanese as a greater part of my spoken environment (and education) while growing up? Sure. I think that if my parents had decided to keep my brother and me immersed in a spoken-Japanese environment, then I think that I would have had a good option of knowing my "other mother tongue." However, I think that my brain is "set up" as a bilingual, however, I've never looked into metrics of bilingualism, such as this recent one:
A. Karmiloff-Smith’s (1990) task of drawing a nonexistent object is considered to be a measure of cognitive ﬂexibility. The notion of earlier emergence of cognitive ﬂexibility in bilingual children motivated the current researchers to request 4- and 5-year-old English–Hebrew and Arabic–Hebrew bilingual children and their monolingual peers to draw a ﬂower and a house that do not exist (N = 80). Bilinguals exhibited a signiﬁcantly higher rate of interrepresentational ﬂexibility in their drawings (e.g., ‘‘a giraffe ﬂower,’’ ‘‘a chair-house,’’ found in 28 of 54 drawings), whereas the level of complex intrarepresentational change was similar across groups. Interrepresentational drawings were previously reported only for children older than 7 years. The speciﬁc mechanisms by which bilinguals’ language experience may lead to interrepresentational ﬂexibility are discussed.Why might I be thinking about this? Well, as much as conservative Republicans might like to mock and denigrate presidential candidates who speak another language (such as mocking Mitt Romney's ability to speak French; Newt Gingrich's ability to speak French is likely waiting in the wings, at least until they find audio or video evidence) as well as so many Americans desiring to make English the official national language of the US (and some people's desire to force new immigrants to assimilate into the English speaking majority).
Never mind that many Americans who can only speak English have been mocked of their inability to write in the language that they want to force others to learn (presumably as well as they know it). On Discover Blogs, "The Crux" wrote about the "Mental Costs of Linguistic Assimilation". Money quote (at least to me):
In the end, it’s clear that English is not under threat in the U.S. —bilingualism is. And the greatest cognitive benefits of bilingualism are likely to be reaped by those who are able to use their non-English language with neighbors, teachers, and bank tellers, those who can talk to other bilinguals in speech that is spangled with Spanglish or Chinglish or Franglais. In other words, those who are lucky enough to have the kind of bilingual environment that my kids never had. Just the kind of environment that English-only advocates seek to obliterate.