OP, you have given a name to my affliction. I have always just referred to it as my "automotive wandering eye."
I've had around 30 cars in 30 years of driving, usually two (or more) at a time. With only a couple of exceptions, I don't sell a car because I don't like it anymore, it's usually just that something newer and shinier comes along.
Currently, I've got two fine cars in my driveway/garage. Both bought new, no problems with either, the RS was a bit of a "dream car" ordered before they were officially announced for the US. I'm in line at my MINI pusher, aka dealer, to get their first GP2 allocation around January to replace the 2011.
And yet....I've had my Audi salesman looking for a suitable used R8 for the past 6 months or so, but the right one at the right price hasn't come along yet. I also found myself this past Sunday perusing the contents of the closed for the day local Porsche dealership. They had several late model CPO 911 variations that put a glimmer in my eye, or I even considered splurging for one fo the new 991's.
One of my oldest and dearest friends told me that evening that "you like buying cars more than you like owning them, the thrill of the hunt." Maybe that's it, but I have gotten to drive some really cool stuff.