This sums it up perfectly. It's the thrill of the chase followed by the satisfaction of a job tastefully done and the simple pleasure of ownership. By way of example, I'm currently getting unnecessarily excited over the imminent arrival of a Mavic groupset with the great help of
@BalkanExpress which will go onto a Vitus 979 to make it 'right'. The bike was a bargain, the groupset not a bad deal, but the finished bike would probably only sell for about what I paid for it, if I chose to sell it. But that's not why I did it - it was for the reasons
@booze and cake describes above, the pleasure of owning functional works of art and The Joy of Fettling (hmmm, gives me an idea for a book...)
Then there's the undisputed pleasure of riding the thing at a pace appropriate to both bike and rider! I love to see classic cars being put through their paces with only a nod to mechanical sympathy, and the same applies to bikes.
Then of course there's the question of what to do with the groupset that comes off the Vitus... and so it goes on.