My sense is that there is a narrow slice of software developers who genuinely do flourish in a pair programming environment. These are people who actually work through their thoughts better with another person in the loop. They get super excited about it and make the common mistake of "if it works for me, it will work for everybody" and shout it from the hilltops.

Then there are the people who program best in a fugue state and the idea of having to constantly break that to transform their thoughts into words and human interaction is anathema.

I say this as someone who just woke up in the wee hours of the morning when nobody else is around so I can get some work done (: