Bringing us back to talk of energy and where e/i may find the source of their energy, stimulus, or motivation (for want of a more definitive word for it) as an introvert I do find certain social contexts invigorating and might go so far as to say give me energy, but others conversely I find quite draining and do require 'me-time' to recharge. Thus to some of my friends I may seem quite sociable and others quite the opposite but that's not to say I find either less enjoyable than the other, more that the approach I have to each and what I take/give to the given situations may vary. This I suppose is due to the size of the social group, or whether it's a different environment than I am used to etc.
Also to what jenocide said; yes, I do think that's definitely part of it and I find myself that I get very little done when I'm in a busy environment, though I do enjoy being alone with music to work and find it can motivate me more than working in silence. I guess it's partly dependent on my frame of mind at the time, and to a certain degree my locale. Living in a busy city generally means there isn't much in terms of silence so it's more replacing noise with music to a degree. Such that if I happen to be in the countryside or a quiet part of London (!) I find it much more relaxing (and at the same time productive) to embrace the silence around me.
Hopefully my 2 cents is relevant or at least slightly made sense! If not, at least you got 2 cents from it!