A bit of both those for me + another - physically unable to ride the model any more.
Limited garage space means that sometimes one has to go to make room for something else - my 350 Bullet made way for a Ducati ST2, as I felt I had done what I wanted to do with the Bullet & fancied an ST2 (only 7 years or so after I test rode one ! ).
The ST2 had to go 3 or 4 years later because of a failed battle to keep modifying the riding position faster than my arms deteriorated. Even with 50mm handlebar risers, riding it brought on tendonitis in my right arm - 2 or 3 weeks recovery after a 3 hour ride isn't really do-able long term, so it had to go.
Similar tale with the Breva 750 - 5 years in the garage, but it had to go because the heavy-ish clutch brought on tendonitis in my left arm !
Others I just didn't get along with (all 4 cylinder bikes, all "Adventure" bikes & the 4 speed 500 Bullet come to mind).
The only one I really regret selling was my Moto Guzzi Mille GT - I convinced myself I "needed" something faster & regretted the decision the next day. Carnells wanted a grand to "undo" the deal (not an insubstantioal sum in 1994), though I did swap one really awful bike (Suzuki GSX1100F) for a "tolerable" one (Yamah XJ900F) under their "7 day exchange" policy for used bikes. The Yamaha went in under a year, though - nice-ish but dull.
I do find increasingly that I have little interest in most moderm motorcycles. It used to be that there was always something that made me want to have a go, if nothing else. Now most seem to be rather samey, rather heavy, rather ugly, impractical (I rather like mudguards that work ! ) & with way more performance than I could use on the road & keep my licence. Must be getting old, I guess.
So many motorcycles to try - so little garage space, money & time to try them !