1. Cluttered thoughts
When I develop software, very little of that time is spent typing. If you were to watch me program, what you'd see is someone sitting in a chair doing nothing. Before I write a single line of code, I'll examine my future program in my head, poking, prodding, trying out different approaches, weighing the benefits, considering edge cases, and so on. I cannot do this if distracted by talking or typing, as it clutters up my thought process (unless the task is quite trivial). Because of this, pair sessions tend to be awkward, with me not saying much and not typing much 90% of the time. What I can do effectively is design a system and then ask someone to critique it before I start coding it out.
2. Social exhaustion
I have a hard enough time dealing with the exhausting process of social interaction as it is. I don't avoid it entirely, because I know that it's important. However, my mental stamina is also important if I'm going to be doing anything useful, so I try to strike a balance between social obligation and isolation. I've never paired for more than a couple of hours, which is draining and becomes stressful before long. I can only imagine that I'd be climbing the walls after 8 hours of that.
3. The zone
I like programming in the zone. Everything becomes so easy and flows so nicely, but it can sometimes take a long time to get into the zone, and it's fragile and breaks easily. I find it impossible to enter the zone if I'm talking.
The exhaustion you get used to. I feel more tired after 8 hours of pairing than 18 of solo.
There seems to be a strong correlation between people pushing pair programming and people I wouldn't want to work with in the first place.