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It's been a little while but I think I remember. The mom left because she had an affair with another man. I don't think that caused her to leave immediately though... It was more to do maybe with the fact that her husband didn't really address that it had happened and that maybe the reason was that there was stagnancy in their relationship and in the whole judgemental Mennonite community in which they were stuck. The sister just grew up and wanted to live somewhere else on her own to be more free and experience more of life. The main character (who is a she not a he)'s bf broke up with her. The reasons aren't entirely clear, she'd been depressed for a long time and not taking care of herself and they were young so he may have just felt that the relationship had run its course. He may have been interested in someone else. The father pretends that he is going off on an adventure but more likely he has gone away to commit suicide. Overall it's a very sad and depressing book despite the sweet humour of the lonely narrator.