Besides the obvious criteria of sentimental value or a gun that was difficult to acquire; why are some guns impossible to part with and others come & go?
I've had Ruger Service-Six's/Security-Six's that were great guns but they didn't scratch that itch. I picked up a Service-Six a few weeks ago I already know that this one is destined for 'Keeper" status. Not sure why. It's not aesthetically perfect and it shoots about as well as the others.