What I've found discourages people from going to sessions is the people who don't know when to join in and when not to. I can think of one Hertfordshire person who can almost empty a session when he turns up, because of that sort of behaviour. I've even heard him playing a different tune to the performer at a singaround because he doesn't like that person.

On person badly out of time, especially a percission instrument, can ruin any tune and that puts others off of bothering to continue to play for ENJOYMENT.