If I don't feel like I can take care of the children safely and effectively, then I will close. It takes a lot for me to get there, but if I literally can't get off the couch, or am running to the bathroom every 5 min I don't think it's good for me to be watching kids. It's one thing to have your own around you, but to have 3-6 toddlers running around, it gets a million times harder.
So, as playfelt said, for me it comes down to coping ability.