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What would you mind someone changing on your computer? In this I mean wallpaper, desktop icons and so on. If you are anything like me, I don’t think you would like very many things to be changed. Even minor, trivial things like the wallpaper would be an unsettling experience to many people. I think it has to do with a users model of control.
When I use the computer I like it to be set up in a familiar way. Windows for one goes a long way to ensure that if you have used one Windows computer, you should be able to use any other without much problem (given use of Explorer as a shell). All desktop environments do this. You can customise superflous things like the wallpaper, but the fundamental workings of the desktop stay the same. The functional aspects of the desktop stay the same.
This doesn't really explain, however, the not wanting to come back to **your** computer and find it has changed in these basic ways, such as the wallpaper. I would say this happens with almost all computer users. If I change the wallpaper on the home PC when I am there, it would be disaproved of. Not in a major fashion, just a feeling of gentle discouragement towards doing it again. This is for my family, who I wouldn't say are anywhere near as protective of their working environment and computer in general as I am.
It comes down to people wanting to feel in control of what they can control. So many things in life we have no control over that it brings a feeling of comfort when we can have some control over the world around us. Thus the simple things like desktop icons and wallpaper are slightly more than just disconcerting when they change.
When Windows crashes and you desktop icons all revert to an auto-arranged layout rather than the way you set them up, nearly **every single person** I have seen gets annoyed with their PC. Nevermind that it takes less than a minute to move them back (especially with the excellent auto-fit to grid of XP). The feeling of control is gone and with it the comfort of having some influence on the world. It's a minor thing in the scheme of things, but minor things build on each other to make bigger things. And the bigger things can have a real effect on us.
In conclusion, one of the things to do as a software developer is to make sure that the programs we do act just like the user expects them to. This produces a better feeling about the product we provide. As well as giving the user more feeling of control, if they like our product they are more likely to choose it over others. Whether open source, free as in beer or commercial, what matters in the end is users. For without users, there would be no point in the software in the first place.