From a very early age, we are taught to break apart problems. This apparently makes complexity more manageable, but we pay a hidden price. We can no longer see the consequences of our actions; we lose our intrinsic sense of connection to a larger whole. When we try to 'see the big picture', we try to reassemble the fragments in our minds, to organise all the pieces. But, as physicist David Bohm says, the task is futile - similar to trying to reassemble the fragments of a broken mirror to see a true reflection. Thus, after a while we give up trying to see the whole altogether.
I think this difficulty emerges when a team tries to adopt only a few Extreme Programming practices at a time. It's understandable why this approach is taken. Most people assimilate new things when they're introduced gradually. Too much, all at once, and people can feel overwhelmed. In my experience, however, when you take this approach teams struggle to pull it all together. They compartmentalise the practices and do not realise the whole that is Extreme Programming. So, if I have a choice, I will always introduce everything in one go and then help people to bring it all together, everyday in all that they do. Certainly it's a challenge for people to learn this way but it's definitely not impossible. And when they succeed it's a measure of their abilities.