As jnovak has noted, I believe bigtwin has hit the nail on the head on this one! It would be great to get a bona fide neuroscientist's take on this. To one degree or another, I believe all serious audio and/or stereophiles are junkies, especially those who are constantly "upgrading" or otherwise tweaking their systems. After a while, we kind of become accustomed to and comfortable with the systems we put together and constantly wonder what would sound better and provide a bigger thrill.
I also believe onhwy61 makes a very valid point here, too. Although I don't have access to data that would validate a statement like "the majority of audiophiles put together a system and run it until it breaks or some major life change intervenes", this certainly rings true for me, personally, and almost all of my audiophile friends. For example, I've only owned 2 turntables in my audiophile life (i.e. a Phillips 212 Electronic that I purchased in 1972, which was still working until I replaced it around 5 years ago). Additionally, I've only owned 3 amplifiers since 1972, still have all of them and all still work fine, although the Sansui 2000x really needs to be recapped. The only two audiophile level components I've ever owned that failed after many, many years of faithful and flawless service were an old Denon cassette deck and a Sony CD player. Generally speaking, good, solidly built components tend to last and perform well for a long time.