Alistair Cockburn points out that traditional processes are based on an assumption that people will successfully accomplish things that they normally struggle with.

In particular, it is assumed that people will change their habits to follow detailed new instructions.  Unfortunately, humans aren’t very good at that!  Old habits die hard and, unlike the computers we program, we don’t reliably follow written instructions.

Perhaps this is why so many process improvement initiatives fizzle out (especially big ones, like adopting RUP).  The new process may be diligently used when first adopted, but then it falls into disuse as people revert to their old habits.

Agile processes are different.  Instead of basing themselves on something humans do badly, agile processes leverage human strengths.  These strengths include the ability to communicate, collaborate, and use initiative.

Agile processes add structure to these natural strengths.  The structure includes tools, techniques and disciples that have proven themselves in real-world software development.