I just tell them exactly why I'm leaving. If it means telling them I don't think the group is skilled enough to make it through in a timely fashion or I've lost interest and want to go do something else. It may seem flakey or rude but I'm not going to do something I don't want to do if it's more time or work than I had planned to go into it. This is a video game and I'm not going to make it suck for myself because of anyone else. It's my leisure time to relax. Not get stressed out I'm wasting time with a group or just 1 or 2 guys who don't want to take it seriously or just too inexperienced.