[JohnBooty]

  For me, it's not necessarily the frequency of random
  conversation/noise/etc, but the lingering feeling that
  you could be interrupted at any time. That idea keeps me 
  from concentrating and working to my full potential.

Yes, this! Also, I hate to admit it, but I haaaate the idea that somebody could walk up from behind me while I'm deep in concentration.

I don't know why. I am not anxious or paranoid in general. I don't fear people walking up behind me when I'm sitting and relaxing, or standing and chatting, or whatever.

It must be some primal thing. My brain must know that if I'm completely engrossed in writing code (as opposed to casually sitting around) then I'm completely defenseless as well, and thus prevents me from full concentration.