Happens more often than I'd like (which would be never).
I'm on an easy run, just minding my own business, then I end up catching up to someone else minding their own business. I pass them, and then notice them running on my shoulder. They typically hold on for as long as they can, which is why I've adopted the practice of maintaining my pace for a couple minutes then burying them. I just want to run at my pace WITHOUT a stranger joining me, and I'd assume they'd rather not endure the humiliation and agony of getting dropped.
Disclaimer: I'm an average runner and I don't have a huge ego, I just don't like people doing this.
What gives?