Sometimes, IRL people piss me off. That way, I can come onto some forums, speak with people with whom I share common interests, can have a laugh and chill. Then, I can calm down and go back to 'reality' with a fresh attitude.
If people have a genuine and immediate reason to leave, they shouldn't EVER have to explain them selves. oky, closure is nice, but if their house burned down, and you were bitching about them for not saying anything, you'd feel so guilty when they came back and told you, wouldn't ya?
If it's a conscious decision to leave, after much deliberation, then someone should just tell at least another member out of courtesy.. but at the end of the day, it's still their choice.