I think I meant to reply to this thread much earlier, but found myself distracted by...God knows what, anymore. Everything.
It's a really fascinating topic. I think [MENTION=17697]small.wonder[/MENTION] is onto something with this theory, but I'm also inclined to think a good portion of instinct can be inborn. It's very possible that I've always been sx-dom; visible demonstrations of such appeared as early as age four or five. A guess at my type back then probably would have held up today, as accurate as ever.
However, I wouldn't argue that my upbringing may have magnified it. Hard to know for sure. Nature and nurture working in tandem, maybe, as with many aspects of personality.
Both of my parents are strong sp-doms, likely so-aux. I felt bonded to my father, but not to my mother. My dad and I have always been pretty close. He's generally more accepting of my eccentricities, communicates in a similar fashion, etc. Most importantly, he has the generalized anxiety that I have (he's a 5w6), and talking with him about it often kept me from feeling like I was going crazy. On the other hand, my mother (2w1) almost exacerbated the feeling that I was out of my mind; she was more likely to scold me for having inappropriate emotions or come off as though she was disapproving because she didn't understand...and had no desire to understand. A "what is wrong with you?" was infinitely more probable than an "are you okay?" and I got used to it.
The catch was that, while growing up, my father traveled a lot for business purposes and was still in a troubled phase of self-medicating his own anxiety to the point of substance dependency. So he was often distant, quite literally, and I was left alone with my mom, who seemed almost afraid of my intensity. We fought all the time. I love both of my parents very much, and their health and happiness has improved along with my own over the years, but the fact is that I had to learn how to self-soothe at a pretty early age. My emotions were always running hot, too, so it was like being really thirsty in the middle of a desert.
I read that Sevens sometimes develop their mental coping habits by fixating on objects of entertainment to distract themselves from real or perceived emotional loss and their fear of continued metaphorical hunger - there's a name for that kind of attachment, I can't remember. But that was totally me as a kid. Just runnin' around with my pile of toys and notebooks, not asking for attention from anyone or anything so long as I could keep myself busy.
On the subject of interpersonal dependency, however, I would not consider myself a codependent person by any stretch of the imagination. That's why some of the descriptions of sx-dom as "one-on-one" really irritate me; some people tend to get the idea that it means an instinctual, desperate search for a soul mate. It feels almost laughably romantic when framed that way, and I tend to disagree with those definitions. I don't find that I need relationships, safety nets, or social frameworks to fuse to; it's not about that.
It's more than obsession, too. It's like having a constant need/want/drive humming away in the back of my head. On a very base, motivational level, I'm mostly concerned with where my next emotional high is coming from. I'm a junkie for intensity.
For what it's worth, I'm also an only child and where I grew up, I had zero neighbor kids with whom to interact. I was very socially isolated - but interestingly enough, I still turned out so-last.
My people skills are solid, but networking with others, actively seeking them out for the sake of staying in the know, mentally mapping the hierarchies of who's who...it just doesn't excite me, and thus it receives the back-burner. It feels exceptionally unimportant in the big scheme of things. Boring.