I love a clean slate. I have gotten in several really bad conflicts with people over the last year or so in which I recommend starting over from a clean machine, they prefer another strategy, and I then refuse to have anything to do with their machines.
This behavior of mine is excessively rigid and decidedly non-agile. Why do I get so upset? Why do I get so upset over other people's computers?
Well, computers do accumulate cruft. (Yesterday a co-worker ran out of disk space on a fairly new macbook pro; he soon discovered that iMovie had been making duplicate copies of all of his videos. 30 gigs worth.) Cruft is nefarious and hard to detect, but it usually slows down the machine. Maybe you installed twenty dashboard widgets and forgot about them. Maybe you installed a system extension that looks up dictionary definitions for all words over eight characters. Maybe you turned on a "run real slow" option somewhere and forgot about it. On the PC, system degradation is often the result of malware. Then in a boiling-the-frog way, system performance degrades... until I sit down at your computer and freak out over how slow it's acting. "I have the same hardware / worse hardware and it's way faster than this." or "This machine is made of nice components; it should be faster than this." That assertion is very difficult to support with hard facts; it relies on my subjective experience of the hundreds of computers I've spent time with in the last twenty years. It doesn't matter if I'm right that the current machine performance is much worse than the hardware's ideal speed; it matters that the frog is just warm but not cooked. A warm frog and a slow-ish computer are not catastrophes for normal people.
Now, I could go off into self-justification here, and claim that I get so upset because of the future of doom that I foresee for a warm frog, and the machine/frog's owners coming stress and regret over a dead pet. That's not it, though. I get upset because I don't know how to save the frog. I don't know how to go from a slow-ish computer to a fast-ish computer. I don't know how to diagnose why a slow-ish computer is slow. My answer is always to start over, wipe the disk, and then the trouble goes away for a while. I think it's kind of fun to get a new environment set up on a clean machine; most people regard setting up a new environment on a clean machine as a major diversion from actually getting work done. And, in truth, people who let me wipe their machines usually end up with a set of new problems; not slow problems, but configuration problems. Most people will experience this as going from a warm frog to a paralyzed (albeit cool) frog... and when considering my recommended course of action, wiping the machine, they foresee the paralyzed frog, a computer which completely prevents them from accomplishing any of their tasks.
This explains the conflicts: we are both predicting doom as a result of following the other's recommendations, but our definition of doom is different. For me, a slowish computer right now is doom; for others, a slowish computer is preferable to an unconfigured computer. We have different values. And it's not my computer so I should keep my frog-related-fears to myself!
[Postscript: This has absolutely nothing to do with my reinventing my entire life every five years.]