What he said.
Every time you use a complex piece of machinery, you walk the road to wearing it out. Motorcycles, electric drills, lawn-mowers - they all start getting worn out. And revolvers...
Every shot tales a toll on a revolver. Light rounds take less strain, heavy loads take more, but every one does something. Under the impact, things stretch, loosen, bend, get a bit out of alignment - ever so slightly, but it's still there. The old car with worn parts analogy is a good one and anybody who's driven an old clunker knows the difference. How long the process takes depends on the revolver and the loads.
As to wearing out a barrel, I can't imagine wearing out a modern steel barrel with lead bullets, not in the lifetime of the average shooter. Super-hot JHPs, maybe, but it would still take a while.
Barrel erosion is caused by extremely high temperatures at extremely high pressure. The .357 Mag can hit 35,000 CUP and that's pushing it. Compared to that, rounds with reputations for short-life barrels are ones like the .220 Swift, with its top 65,000 CUP, almost twice as high. The .30-06 - with no real reputation for short barrel life, tops out IIRC at around 50,000 CUP.
For what it's worth, tank gun barrels used to be limited to something around 100 shots with armour-piercing sabot. I saw a sectioned 20-pounder barrel in a tech museum in England - the chamber end of the rifling looked like it had been cast in clay and then had a hose turned on it. It was as if the steel had flowed away.
