Chain Wear

[Süsser Tod]

Last Saturday I replaced the chain on the XT660. The old one was stretched, the rollers were kinda lose and had too many stiff links.

I know that when a chain stretches it's not because of the links stretching, but from the wear on the pins, then you also have wear on the rollers themselves that will give the chain a bit of play over the sprockets, that's clear to me...

What is the cause for stiff links? Chain being too lose or too tight? I don't think it's lack of lubrication, as the lube on an o-ring chain prevents rust and wear from the sprocket/chain contact, but it doesn't do anything to lubricate the pins and rollers. I've also read somewhere else that a stiff link can be caused by the failure of an o-ring, which lets the factory lube escape and then the link/roller/pin seizes. BUT, then why does a non-o-ring chain develops stiff links just like an o-ring chain? It shouldn't, all links should be equally stiff or lose.