Interesting question. I’ve bought and sold a few bikes over my time, the only one that I wish I hadn’t, and the bike I had the longest actually, was the Cannondale Beast of the East. I would still be using that now if I hadn’t got rid of it. I loved that bike. I occasionally check out ebay for one when I have a little spare change.
Other than that, I have a frame (GT Arrowhead) that I bought off here, built up, stripped down (but for some reason kept), built up, stripped down. I’m now taking the paint off it so I can run it naked.
Other bikes are tools that keep me interested or get disposed of. I had a steel Cindercone that I crashed in Cannock. I bent the forks, can’t remember why but I left the bike at a guy’s flat in Stoke, never saw it again. I got a road bike, didn’t like it so sold it on the classifieds for the cost of the wheels I’d put on it.
TBH, I get just as much enjoyment from fiddling and faffing with bikes as I do from riding them, so getting rid of a bike and getting new bits is really exciting for me.