Why you can only change a DVD drive's region a limited number of times? Because that's the way the Hollywood studios want it. They like to control how their product is released in different parts of the world.
Twenty years ago, American cinephiles often bought imported Japanese Laserdiscs because they were superior to American offerings (my son first saw Disney's 20000 Leagues Under the Sea with Japanese subtitles). But the studios wanted to control when and how a given film was released in different parts of the world, so they insisted on regional codes as part of the DVD standard, and later for the Blu-ray standard, as well.
North America is Region 1 for DVDs, and part of Region A for Blu-ray. Most set-top DVD players are built for a particular region, and cannot play discs encoded for another region (they can all play region-free discs, however). Region-free players are also available.
PC DVD drives are more versatile then set-top players, with the region assignment built into the software. You generally can change the region of a drive four times in the drive's lifetime. In Windows 7 or Vista, click Start, type device manager, and press ENTER. In XP, select Start, then Run; type devmgmt.msc, and press ENTER.
Once there, expand the DVD/CD-ROM drives section, then double click your drive. Click the DVD Region tab.