Over the years I've known a few gentlemen as you describe, they work for a few years perfecting a partciular car, investing a large amount of time, energy and family budget into the project, and when it's done, they take it to a few shows to proudly display their creation, then lose their enthusiasm for it. They sell it, to recoup part of their investment, and use the money to fund the next "project".
Some of the guys I've known will be planning the next project, before they finish the one at hand. Then it becomes a matter of hurrying up to finish the first project so they can begin the next. They have a mental list of car projects they would like to accomplish, the list is continually being edited as they change their mind about a particular car, or see a new one to add to the list. They are always on the look out for donor cars for their projects, and will shift priorities when they come across a deal they couldn't pass up.
For these guys the joy they get from ownership is working on it. When the work is done, so is the fun. To finish a project, is to get get that particular car "out of their system" so they can move on. You will find this is not that unusual in the auto enthusiast world. That personality is just one of the many types of personalities you'll find amongst our ranks. No better or worse than anybody else, just different. Different is a kind way to describe any of us.
With regards to Panteras, if you subscribe to PI magazine you can sometimes predict when this is going to happen, because owners will submit stories about their cars to Dave Adler just prior to putting them on the market. Once the magazine prints the story about their car, it goes up for sale shortly thereafter.
your friend on the DTBB