Why did they come back and then leave again?

Coming back is often about relief: the comfort of being wanted again, or an escape from loneliness. Once that feeling is restored, the things that made them leave the first time are still there, and nothing has been worked through. So they leave again, often faster. It can feel like you failed a second test, but the pattern was set before they returned, and the lesson is in the pattern rather than in anything you did.