Aside from one's subjective take on pair programming -- and there are obviously many valid reasons why it doesn't work for some folks -- I'd have to guess that doing it 100% of the time is quite inefficient. Software development presents a wide variety of tasks of varying cognitive difficulty. I tend to believe that pair programming's sweet spot is for tasks of medium to medium-high levels of difficulty.
For the most challenging tasks, say, coming up with an original solution to an unsolved problem, I think it's better to alternate deep focused thinking alone with group brainstorming. But you really need that meditative alone time to gnaw at difficult problems -- for me it tends to be in the shower, or during a walk. That's where you have those awesome "Aha!" moments.
On the other end of the scale, certain mundane/repetitive/trivial tasks really don't warrant pair programming IMHO. You're spending two brains on something that really takes 1/4 of a brain.
The ideal case for pair programming, IMHO, are those tasks where you can visualize the general path forward, but where there are significant challenges wrt implementation such that another pair of eyes can really provide insight. For example, say you have to implement a module, and you have a general idea of what it needs to do, but the overall decomposition and abstractions involved may be inchoate. While I think a fair amount of "implement this new feature" type of stories fall into that category, in my experience that comprises only ~40-60% of the total workload.
The above mostly applies to pair programming with two fairly equivalent/experienced developers. I agree with others who have noted that it's a fantastic training mechanism for new/junior devs. And then, there are various special case scenarios that justify it, as in, you have a problem with X and need to pair with an expert in X to resolve it. IMHO, these are best done ad hoc rather than via a formal process.
For the most challenging tasks, say, coming up with an original solution to an unsolved problem, I think it's better to alternate deep focused thinking alone with group brainstorming. But you really need that meditative alone time to gnaw at difficult problems -- for me it tends to be in the shower, or during a walk. That's where you have those awesome "Aha!" moments.
On the other end of the scale, certain mundane/repetitive/trivial tasks really don't warrant pair programming IMHO. You're spending two brains on something that really takes 1/4 of a brain.
The ideal case for pair programming, IMHO, are those tasks where you can visualize the general path forward, but where there are significant challenges wrt implementation such that another pair of eyes can really provide insight. For example, say you have to implement a module, and you have a general idea of what it needs to do, but the overall decomposition and abstractions involved may be inchoate. While I think a fair amount of "implement this new feature" type of stories fall into that category, in my experience that comprises only ~40-60% of the total workload.
The above mostly applies to pair programming with two fairly equivalent/experienced developers. I agree with others who have noted that it's a fantastic training mechanism for new/junior devs. And then, there are various special case scenarios that justify it, as in, you have a problem with X and need to pair with an expert in X to resolve it. IMHO, these are best done ad hoc rather than via a formal process.