the number one reason is simply software. People install different things, and they use different versions of itunes to get stuff on there. Problematic software interactions, memory retention, process handler, etc are the number one source of problems.
If we were all running the exact same software problems would be less likely, however user A might just browse the web, while user B opens 3 streaming programs, skype, facetime, gps software, and tries to crunch panoramas before his locks up and looses his open safari tabs. Both wonder why A has no problems and B has constant problems.